you cant have two accounts, please let me know which you would like to keep.
as for your mod, if using CG/2 you can just ground the signal. so, one leg of your momentary switch will go to a common ground, and the other leg can go to the via of each button, that way you wont have to disturb the pads keeping the original function of the button.
In the resource center you will find controller scans with the vias for each button marked... (thanks RDC)