AFAIK, you cannot even add rows to the bottom (or columns to the right) of an existing setup. That's because any existing setup is not visible in the editor and you *always* have to restart from scratch whatever change you want to do.
A revamped multiple map setup editor seems to be already in the list of features that will be in Vassal 4 (like a wish list for 3 years from now Santa, though

) , see thread from 1-2 weeks ago (in Module Design section, I think).
In the meanwhile (i.e. Vassal 3.1-2) it would be nice to know if one can manually edit the buildfile to access exsiting multiple board setups and make changes directly there. Any clue anybody?