"Finally the mix may go to another out synth."

If by that you mean you are using an external synth, you would have to record that synth to its own (audio) tracks.

Then it's just a matter of clicking "Audio - Merge Audio and Dxi tracks to a stereo wave file" command.
This gets you a 'mix'. Then you can open that wave file in RB to do any mastering (trim start/end point, adjust levels, add any mastering FX, etc).

I do not work here, but the benefits are still awesome
Make your sound your own!