I don't think you can do that (easily--anythings possible with enough money) with the factory head unit. I know that SONY has some kind of source selector but I'm pretty sure that it's only compatible with other SONY products, imagine that.
if you aren't too picky on sound quality there are cd changers produced that transmit a signal to your radio via FM. of course the music will only be 'radio quality,' but since you aren't going to use it forever...maybe. I am personally very picky on my musics quality.
or maybe you can find some kind of adapter that adds an auxilary input to the CD changer input to allow you to keep the CD changer and have an auxilary unit hooked up also.
on a side note, nice car, man. congratulations.