M2M relationships in PowerPivot will most likely cause more headaches than solutions unless you are aware of what you are doing. After some thought, I realized that I really do not want an m2m relationship because it would result in junk data for what I wanted. I just wanted to start my answer off acknowledging that I did not achieve an m2m.
So if you want to link SharePoint data to Excel here is what you need to do:
Export SharePoint data as an RSS feed. If this is something others will need to refresh then the rss file will need to be stored on a shared drive.
Add PowerPivot if you have not already done so. In PowerPivot get external data from other sources (rss feed) then add your file. To link data you will need a Unique ID to join tables on.