diff --git a/src/finalisers/iife.js b/src/finalisers/iife.js
index f39d00f5855..f16618a33e9 100644
--- a/src/finalisers/iife.js
+++ b/src/finalisers/iife.js
@@ -26,17 +26,17 @@ const thisProp = name => `this${keypath( name )}`;
 export default function iife ( bundle, magicString, { exportMode, indentString, intro, outro }, options ) {
 	const globalNameMaker = getGlobalNameMaker( options.globals || blank(), bundle, 'null' );
 
-	const name = options.moduleName;
+	const { extend, moduleName: name } = options;
+	const isNamespaced = name && ~name.indexOf( '.' );
+	const justVariable = !extend && !isNamespaced;
 
-	if ( name && !isLegal(name) ) {
+	if ( name && justVariable && !isLegal(name) ) {
 		error({
 			code: 'ILLEGAL_IDENTIFIER_AS_NAME',
-			message: `Given moduleName - ${ name } - is not legal JS identifier.`
-		})
+			message: `Given moduleName (${name}) is not legal JS identifier. If you need this you can try --extend option`
+		});
 	}
 
-	const isNamespaced = name && ~name.indexOf( '.' );
-
 	warnOnBuiltins( bundle );
 
 	const external = trimEmptyImports( bundle.externalModules );
@@ -50,7 +50,7 @@ export default function iife ( bundle, magicString, { exportMode, indentString,
 		});
 	}
 
-	if ( isNamespaced ) {
+	if ( extend ) {
 		dependencies.unshift( `(${thisProp(name)} = ${thisProp(name)} || {})` );
 		args.unshift( 'exports' );
 	} else if ( exportMode === 'named' ) {
@@ -63,7 +63,7 @@ export default function iife ( bundle, magicString, { exportMode, indentString,
 	let wrapperIntro = `(function (${args}) {\n${useStrict}`;
 	const wrapperOutro = `\n\n}(${dependencies}));`;
 
-	if ( exportMode !== 'none' ) {
+	if ( exportMode !== 'none' && !extend) {
 		wrapperIntro = ( isNamespaced ? thisProp(name) : `${bundle.varOrConst} ${name}` ) + ` = ${wrapperIntro}`;
 	}
 
diff --git a/src/rollup.js b/src/rollup.js
index 68c532f9943..8daa37c0e4e 100644
--- a/src/rollup.js
+++ b/src/rollup.js
@@ -19,6 +19,7 @@ const ALLOWED_KEYS = [
 	'dest',
 	'entry',
 	'exports',
+	'extend',
 	'external',
 	'footer',
 	'format',
diff --git a/test/form/extend-exports/_config.js b/test/form/extend-exports/_config.js
new file mode 100644
index 00000000000..732195b3125
--- /dev/null
+++ b/test/form/extend-exports/_config.js
@@ -0,0 +1,7 @@
+module.exports = {
+	description: 'extends module correctly',
+	options: {
+		extend: true,
+		moduleName: 'foo'
+	}
+};
diff --git a/test/form/extend-exports/_expected/amd.js b/test/form/extend-exports/_expected/amd.js
new file mode 100644
index 00000000000..8e845b5433b
--- /dev/null
+++ b/test/form/extend-exports/_expected/amd.js
@@ -0,0 +1,9 @@
+define(['exports'], function (exports) { 'use strict';
+
+	const answer = 42;
+
+	exports.answer = answer;
+
+	Object.defineProperty(exports, '__esModule', { value: true });
+
+});
diff --git a/test/form/extend-exports/_expected/cjs.js b/test/form/extend-exports/_expected/cjs.js
new file mode 100644
index 00000000000..316e49a5c1b
--- /dev/null
+++ b/test/form/extend-exports/_expected/cjs.js
@@ -0,0 +1,7 @@
+'use strict';
+
+Object.defineProperty(exports, '__esModule', { value: true });
+
+const answer = 42;
+
+exports.answer = answer;
diff --git a/test/form/extend-exports/_expected/es.js b/test/form/extend-exports/_expected/es.js
new file mode 100644
index 00000000000..b460392dbdd
--- /dev/null
+++ b/test/form/extend-exports/_expected/es.js
@@ -0,0 +1,3 @@
+const answer = 42;
+
+export { answer };
diff --git a/test/form/extend-exports/_expected/iife.js b/test/form/extend-exports/_expected/iife.js
new file mode 100644
index 00000000000..5cddbe0c992
--- /dev/null
+++ b/test/form/extend-exports/_expected/iife.js
@@ -0,0 +1,8 @@
+(function (exports) {
+	'use strict';
+
+	const answer = 42;
+
+	exports.answer = answer;
+
+}((this.foo = this.foo || {})));
diff --git a/test/form/extend-exports/_expected/umd.js b/test/form/extend-exports/_expected/umd.js
new file mode 100644
index 00000000000..55b0d791f77
--- /dev/null
+++ b/test/form/extend-exports/_expected/umd.js
@@ -0,0 +1,13 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory(exports) :
+	typeof define === 'function' && define.amd ? define(['exports'], factory) :
+	(factory((global.foo = global.foo || {})));
+}(this, (function (exports) { 'use strict';
+
+	const answer = 42;
+
+	exports.answer = answer;
+
+	Object.defineProperty(exports, '__esModule', { value: true });
+
+})));
diff --git a/test/form/extend-exports/main.js b/test/form/extend-exports/main.js
new file mode 100644
index 00000000000..64a32fd291e
--- /dev/null
+++ b/test/form/extend-exports/main.js
@@ -0,0 +1 @@
+export const answer = 42;
diff --git a/test/form/extend-namespaced-exports/_config.js b/test/form/extend-namespaced-exports/_config.js
new file mode 100644
index 00000000000..fbd21b3f636
--- /dev/null
+++ b/test/form/extend-namespaced-exports/_config.js
@@ -0,0 +1,7 @@
+module.exports = {
+	description: 'extends namespaced module name',
+	options: {
+		extend: true,
+		moduleName: 'foo.bar.baz'
+	}
+};
diff --git a/test/form/extend-namespaced-exports/_expected/amd.js b/test/form/extend-namespaced-exports/_expected/amd.js
new file mode 100644
index 00000000000..8e845b5433b
--- /dev/null
+++ b/test/form/extend-namespaced-exports/_expected/amd.js
@@ -0,0 +1,9 @@
+define(['exports'], function (exports) { 'use strict';
+
+	const answer = 42;
+
+	exports.answer = answer;
+
+	Object.defineProperty(exports, '__esModule', { value: true });
+
+});
diff --git a/test/form/extend-namespaced-exports/_expected/cjs.js b/test/form/extend-namespaced-exports/_expected/cjs.js
new file mode 100644
index 00000000000..316e49a5c1b
--- /dev/null
+++ b/test/form/extend-namespaced-exports/_expected/cjs.js
@@ -0,0 +1,7 @@
+'use strict';
+
+Object.defineProperty(exports, '__esModule', { value: true });
+
+const answer = 42;
+
+exports.answer = answer;
diff --git a/test/form/extend-namespaced-exports/_expected/es.js b/test/form/extend-namespaced-exports/_expected/es.js
new file mode 100644
index 00000000000..b460392dbdd
--- /dev/null
+++ b/test/form/extend-namespaced-exports/_expected/es.js
@@ -0,0 +1,3 @@
+const answer = 42;
+
+export { answer };
diff --git a/test/form/extend-namespaced-exports/_expected/iife.js b/test/form/extend-namespaced-exports/_expected/iife.js
new file mode 100644
index 00000000000..1b21487c31b
--- /dev/null
+++ b/test/form/extend-namespaced-exports/_expected/iife.js
@@ -0,0 +1,10 @@
+this.foo = this.foo || {};
+this.foo.bar = this.foo.bar || {};
+(function (exports) {
+	'use strict';
+
+	const answer = 42;
+
+	exports.answer = answer;
+
+}((this.foo.bar.baz = this.foo.bar.baz || {})));
diff --git a/test/form/extend-namespaced-exports/_expected/umd.js b/test/form/extend-namespaced-exports/_expected/umd.js
new file mode 100644
index 00000000000..aa4c0b0562b
--- /dev/null
+++ b/test/form/extend-namespaced-exports/_expected/umd.js
@@ -0,0 +1,13 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory(exports) :
+	typeof define === 'function' && define.amd ? define(['exports'], factory) :
+	(factory((global.foo = global.foo || {}, global.foo.bar = global.foo.bar || {}, global.foo.bar.baz = global.foo.bar.baz || {})));
+}(this, (function (exports) { 'use strict';
+
+	const answer = 42;
+
+	exports.answer = answer;
+
+	Object.defineProperty(exports, '__esModule', { value: true });
+
+})));
diff --git a/test/form/extend-namespaced-exports/main.js b/test/form/extend-namespaced-exports/main.js
new file mode 100644
index 00000000000..64a32fd291e
--- /dev/null
+++ b/test/form/extend-namespaced-exports/main.js
@@ -0,0 +1 @@
+export const answer = 42;
diff --git a/test/form/module-name-scoped-package/_config.js b/test/form/module-name-scoped-package/_config.js
index 56818817923..5d2d240e9d7 100644
--- a/test/form/module-name-scoped-package/_config.js
+++ b/test/form/module-name-scoped-package/_config.js
@@ -1,6 +1,7 @@
 module.exports = {
 	description: 'allows module name with dashes to be added to the global object',
 	options: {
+		extend: true,
 		moduleName: '@scoped/npm-package'
 	}
 };
diff --git a/test/form/module-name-wat/_expected/iife.js b/test/form/module-name-wat/_expected/iife.js
index 2f7b7b24015..5d377d98182 100644
--- a/test/form/module-name-wat/_expected/iife.js
+++ b/test/form/module-name-wat/_expected/iife.js
@@ -1,11 +1,11 @@
 this.foo = this.foo || {};
 this.foo['@scoped/npm-package'] = this.foo['@scoped/npm-package'] || {};
 this.foo['@scoped/npm-package'].bar = this.foo['@scoped/npm-package'].bar || {};
-(function (exports) {
+this.foo['@scoped/npm-package'].bar['why-would-you-do-this'] = (function (exports) {
 	'use strict';
 
 	let foo = 'foo';
 
 	exports.foo = foo;
 
-}((this.foo['@scoped/npm-package'].bar['why-would-you-do-this'] = this.foo['@scoped/npm-package'].bar['why-would-you-do-this'] || {})));
+}({}));
diff --git a/test/form/module-name-with-dashes/_config.js b/test/form/module-name-with-dashes/_config.js
index c466b887cd3..5a5ec8669fa 100644
--- a/test/form/module-name-with-dashes/_config.js
+++ b/test/form/module-name-with-dashes/_config.js
@@ -1,6 +1,7 @@
 module.exports = {
 	description: 'allows module name with dashes to be added to the global object',
 	options: {
+		extend: true,
 		moduleName: 'module-name-with-dashes'
 	}
 };
diff --git a/test/form/namespaced-named-exports/_expected/iife.js b/test/form/namespaced-named-exports/_expected/iife.js
index 69637e91324..4043b2692f9 100644
--- a/test/form/namespaced-named-exports/_expected/iife.js
+++ b/test/form/namespaced-named-exports/_expected/iife.js
@@ -1,10 +1,10 @@
 this.foo = this.foo || {};
 this.foo.bar = this.foo.bar || {};
-(function (exports) {
+this.foo.bar.baz = (function (exports) {
 	'use strict';
 
 	var answer = 42;
 
 	exports.answer = answer;
 
-}((this.foo.bar.baz = this.foo.bar.baz || {})));
+}({}));
diff --git a/test/test.js b/test/test.js
index dfc78072346..d1c0447fbbd 100644
--- a/test/test.js
+++ b/test/test.js
@@ -136,7 +136,7 @@ describe( 'rollup', function () {
 			return rollup.rollup({ entry: 'x', plUgins: [] }).then( () => {
 				throw new Error( 'Missing expected error' );
 			}, err => {
-				assert.equal( err.message, 'Unexpected key \'plUgins\' found, expected one of: acorn, amd, banner, cache, context, dest, entry, exports, external, footer, format, globals, indent, interop, intro, legacy, moduleContext, moduleName, noConflict, onwarn, outro, paths, plugins, preferConst, pureExternalModules, sourceMap, sourceMapFile, targets, treeshake, useStrict, watch' );
+				assert.equal( err.message, 'Unexpected key \'plUgins\' found, expected one of: acorn, amd, banner, cache, context, dest, entry, exports, extend, external, footer, format, globals, indent, interop, intro, legacy, moduleContext, moduleName, noConflict, onwarn, outro, paths, plugins, preferConst, pureExternalModules, sourceMap, sourceMapFile, targets, treeshake, useStrict, watch' );
 			});
 		});