Hi Stevec with Mixbus you have an app called Jack Audio.
You can route the Sonar stems into Mixbus like a tape recorder.
From Harrison site:
What is JACK?
JACK is a powerful system service that allows applications to route audio between each other, the soundcard, and even the internet. On Linux, JACK is pre-installed on systems with realtime audio performance. On OSX, JACK is installed on the system as part of the Mixbus installation process. On Windows, a simplified version of JACK is installed for use by Mixbus, which other applications do not see. Please note that the user does not have to aware of JACK. Mixbus performs all of the setup and operation of the JACK server using its "Audio Setup" dialog, and handles audio routing using its own methods.
Best