Well, you would have to WIRE them, but yes, you could simply add buttons to you current panel. You may or may not have to do at least some soldering or wire splicing, it depends on how it is currently set up.
If your cabinet is already a Jamma cabinet then a simple Capcom kick harness and the extra buttons should be all you need. Might have to run the cab ground over to the new buttons as well, as I can't recall if the kick harness has a ground on it or not.
Or, if it is already a Mame, then wire your new buttons the same way you wired your old ones.