I'm not sure you can get adapters, though hopefully someone will prove me wrong!
I have the exact opposite problem. My car came with a pioneer system- headunit and 6CD changer and i've replaced it with an Alpine headunit and now the changer is useless!
In fact i've been looking at the Alpine CHA-S634 as a replacement as i can't find an adapter lead.
I don't expect you'd be interested in the pioneer changer i have as it's a few years old and not MP3 compatible, but let me know if you are as it's no use to me and not worth much!
Good luck with the hunt and post back any positive results
