I happen to know a very easy way to do it, and I've been doing it this way since Sims 1.0
All you do is copy the songs you want and then you paste them into whichever music folder you want which are all in here -> My Documents\EA Games\The Sims 2\Music
That's the music folder and then there are a bunch of seperate ones for each category. Build mode, buy mode, Neighborhood, CAS (Create a Sim mode), and then all the different radio stations so you can put your own rock, pop, rap, etc. :-)
Thank you very much