The standalone nature of Reason makes it very handy to use as a 'sketch pad', on a laptop or whatever. You can link it to other programs via Rewire (I think that's what it's called), import MIDI, or use Recycle, but otherwise you can't bring in external tracks. However, it's fairly easy to export Reason MIDI and audio into other programs that are more versatile and/or feature-rich. That's what I usually do.
The Rewire approach might work for you. You can Rewire into Cubase, Protools, Reaper, etc.
i'm actually speaking of controlling an external, hardware synth via midi - though there ought to be a way to implement this. hmmm