I haven't done this yet myself, but do intend to try it, but I'm assuming it's the usual way of making a memory stick up with the usual PS3 folder and naming a folder within it Music. And then putting the MP3's in there.
The only thing is I don't know if you can actually 'add' music to the existing playlist, I'm pretty sure that you'll choose your own playlist and that will take the place of the exisiting one. I could be wrong, but just assume this would happen.
FD