Dave, the safest bet is to ignore the major chords, and play two adjacent bass notes for the minors.

It may sound better if you blank off the lower bass notes (there's usually two per bass on smaller accordions) with a length of card between the reed block and the soundboard - easily reversible if you don't like it.

Tinkering with the linkages may work okay, though - quite often all you need to do is carefully spring the require rod past the little bar that sticks out from the key - try it with one, and if it works, the world's your lobster... again, it's easily reversible if you don't break anything!!