another battery question

i have a 1991 club car ds and did the conversion with a 500 amp controller and 400 amp sel. I have new 36 volt batteries and a new charger, I was curious if I could temporarily hook up an extra 12 volt battery just to see if it would be worth it changing it all over to 48volt I was thinking I could run a marine 12 volt I have in the bag storage and if I can how would I hook it in with the other batteries
thanks