Not really, you'll still need to have the expansion loaded on your PS3 as well, but you wont need another code. In other words, yes, you can buy the PC expansion, install it and register it on the PC, and then all you'll have to do on your PS3 is install the PS2 version (can borrow a copy from a friend or something instead of buying it) and you'll be all set on both systems...
I have a feeling that's not exactly what your trying to do though...