That's called Word Mini Toolbar
(naming is important to get the right search results - tip, always find adequate names:).
Anyway I think you're out of luck, I've never seen anything related to that (I know how you can change the context menu, that's no problem). And frankly it doesn't seem very likely, as that seems like a 'cute little' toolbar that's specifically formatted to fit small format, size/looks.
Check this MS link Developer Overview of the User Interface for the 2007 Microsoft Office System
And somewhere inside it says
You cannot modify the Mini toolbar programmatically.
And this question/answers confirm that pretty much
MS Word Plugin, Adding a button which pops up on right click on selected text