I believe the code requires that a ground wire be present if a 3 pronged outlet is to be installed. You'll have to take one outlet out and see what type of wires you have in there. If a ground wire is present then you can replace the outlet with a 3 pronged type, and connect the bare wire to the ground screw on the outlet. If not, you'll have to figure out a way to run a ground wire to the panel. If you have metal conduit electric, then a ground wire may not be needed.
You could also replace the outlet with a GCFI which doesn't require a ground wire, but you still won't have a true ground.