I don't have much experience in this area yet, but have general
familiarity as I built a protools computer for a client.
This article may be of interest to you about ReWire:
Also another solution I saw was to export your music to a high
quality (good sample rate) WAV etc. then import it into protools
& mix with your audio.
[url]http://www.refusesoftware.com/refuse.html[/url] (if using OS 9)
The following articles look like they might really point you
in the right direction...
Propellerhead and digidesign both have forums as well you can utilize
if these links don't show exactly the way you want to do it...