You can charge an 8 volt battery with a 12 volt charger. It's similar to putting an equalizing charge on a 12 volt battery.
Two things to watch. First, the charging current. You can start on the normal 10 amp range, but be prepared to reduce the charging rate if the battery draws more current than the charger can supply. What will normally happen is the charger's transformer will overheat and the thermal overload will trip, shutting it down until things cool off. If the battery is completely dead you'll see little to no charging current at first, then it'll increase dramatically as the charge gets going.
It shouldn't take long to put enough of a charge into the battery to let the regular charger pick up - no more than a half hour or so. If you want to put a full charge into the battery via the 12 volt charger, watch the end of the charging cycle. Unlike charging a 12 volt battery the charging rate won't taper off at end of charge, it'll just make the battery outgass a lot and get hot. Shut off the charge before it reaches this point.
Charge each battery individually, not more than one at a time.
Another thought is to connect the 12 volt charger across the entire 48 volt battery bank, turn it on and then connect the cart's main charger. The 12 volt charger may supply enough voltage to fool the cart's charger into picking up - when it does, disconnect the 12 volt charger.