In theory, you can edit it using vba, but I've never attempted it. I use Word 2003 to edit such toolbars. This is difficult because it requires that dotm formatted documents be saved in .dot format, losing current features, and then converted back, reintroducing those.

I join Graham in urging you to explore custom Ribbon modification (not through the User Interface, but through XML). Greg Maxey's page is the best place to start. His site has many more pages on particular issues with Ribbon customization.

If you want a reference book, I recommend RibbonX: Customizing the Office Ribbon by Robert Martin, Ken Puls, and Teresa Hennig.