I built some content for the Sims 1, original figures in fact.

I've looked into doing the same for the Sims 2, but from what I've found out that would be very, very dificult.  I think they went out of their way to make it so only they could provide content, the files are in binary form, all compresed into into a single file.  There are some tools available but it isn't going to be easy to do anything other than what they want you to do.  As Chelonov said, it is very complicated.

Sort of  a shame, really.   I only bought the game to see if I could work with it.