http://dotnetdud.blogspot.com/2010/08/video-tutorial-add-new-button-on-office.html

Video Tutorial - Add a new button on Office backstage using C#

How to add a new button to Office (Excel) Backstage from Addin using VSTO (C#)

The following XML adds a new button on Backstage. The button will be added at the end as we didn't specify a location.