We're currently working on a Microsoft Word add-in in my day job. The company already has an Outlook add-in and they wanted to use the same manifest file for the Word add-in. The manifest files have some keys that look similar. Actually, most of the content seems similar, but it's not possible to merge these two.
There are three types of Office add-ins: Task pane, Content and Mail. These have different schema definitions at the top of the file. So it's not possible to combine these three different types of Office add-ins. If an add-in needs mail permissions, it cannot work in Word, and if it requires document permissions, it cannot work in Outlook.