Until now, Ad extensions tab help you manage and track your campaign extensions in AdWords. Now, it lets you "share extensions" across multiple campaigns.
What's more, if you need to change part of your extension later, you only need to change the extension once and all campaigns sharing that extension will be automatically updated.
Any extension you add is automatically available for sharing between campaigns. To add a shared extension to a campaign, visit the Ad extensions tab for the destination campaign. Then, select the extension type from the drop down box, and click 'New extension.'
You'll see the option to choose one of your existing extensions from another campaign, or to create a new extension. Choose the extensions you'd like to use, click save, and you're done.
More Info: AdWords help center