ReWire? Depends on whether the music already exists as a media file that Reaper can open. There's no "media pool" as such - just the media explorer window, from which you can drag files into the waveform window or just double click on them.
I have not used rewire in Reaper yet but I gather from other like programs that you just rewire and record in Reaper when you have it finalised.
That way you end up with all your sound files in Reaper. For those you are still composing or working on just do not record them into Reaper until you are ready to do so.