Generally, a 3-prong outlets work on 2-wire installation just as well. The 2-wire installation is bound to have one of the wires grounded (the neutral), so the grounding prong is just connected to the neutral, together with one of power prongs. The key is choosing the correct one!. The only thing you lose on 2-wire instalation is the option to install a central GFCI.
Now, the legality of replacing 2-prong sockets with a 3-prong ones depends on local code, and that's why you should call a licensed electrician. If the operation is possible, it won't be expensive as it's merely replacing a few sockets. A licensed electrician will know if such retrofit is possible without any extra conditions, on condition that the new sockets have integrated GFCI or not possible at all in your installation.
//edit: Please do not change the meaning of my post. The first part is not an advice, it's merely a summary on how installations were done in pre-GFCI times. The only advice here is to hire a licensed electrician.