Custom UI Editor Tool
One of the ways to extend the MS Office ribbons is to insert an XML part into an Office Open XML document that defines your custom UI. With this approach, you can add new groups and controls, hide existing ribbon elements, and define callbacks to handlers that you write in managed code: http://openxmldeveloper.org/blog/b/openxmldeveloper/archive/2006/05/26/customuieditor.aspx.