Your best bet would be these two MSDN articles: Customizing the 2007 Office Fluent Ribbon and Creating a Custom Tab by Using Ribbon XML, although they look like they focus on doing it across all applications, rather than just one. There help docs for each Office app, to get them all try this Google query: site:msdn.microsoft.com office 2007 add button to ribbon.
This snippet, from the first link, should be what you're looking for:
What About Existing Solutions?
In previous versions of Office, developers used the command bars object model to build the Visual Basic code that modified the Fluent UI. In the 2007 release of Office, this legacy code continues to work in most cases without modification. However, changes made to toolbars in Microsoft Office 2003 now appear on an Add-Ins tab. The type of customization that appears depends on the original design of the add-in. For example, Office creates a Menu Commands group that contains items added to the previous menu structure (such as the File menu, the Insert menu, and the Tools menu). Office also creates a Toolbar Commands group that contains items added to the previous built-in toolbars (such as the Standard toolbar, the Formatting toolbar, and the Picture toolbar). In addition, custom toolbars that are added by an add-in or document appear in the Custom Toolbars group on the Add-Ins tab.