There is no reason why you need to bounce the entire mix in Reason to a WAV file. You can solo each part and bounce each to an individual .wav file. Just make sure that you don't alter your project start and end markers, so that all the .wav files are the same length. Then you can import all those .wav files onto individual tracks in Sonar and do your mixing there. Needless to say, I'd bounce those wav files dry with no fx, eq or compression on them.
Then mix your tune ensuring everything is at reasonable levels.