Sonar le record drum track

I'm not sure I understand what you're trying to do...
You import AUDIO tracks from the BR800.  This should be straightforward.  Import the WAV, and click SAVE...
What is your "synthetic drum" track.  Audio?  MIDI?  Is this what you're trying to save?  If it's audio, it should be the same as what you do with the BR800 (or any other audio, the source is irrelevant).  If it's MIDI, are you running it through a soft-synth, or out to an external synth?

So you're programming a MIDI track to feed the drum synth.
If you can hear it, and it plays while you're working on it, simply saving the Sonar project should suffice.
If this isn't working, I'm not sure what you mean by "save it as an additional track."  If you want to save it as an audio track, you'll need to select both the MIDI and Synth tracks and Bounce to Track.  This will give you an audio (WAV) file.  Then Save.

Then it sounds like Bounce To Track is what you want.

welcome to Sonar. It's a little confusing trying to figure out what you want to accomplish.
but that's okay because I typically don't understand what people are saying .
 
But what I gather is, you imported 3 audio tracks from your Boss (a digital 8 track)
and now want to add a drum track to it using Sonars soft synths.
 
So, you already have your music on tracks 1-3.
what you want to do is use the "Insert Synth" option at the top of the screen.
you insert the synth you want to use, but its not going to be audio (unless you set it up like that)
instead, its going to be a midi track. Which is, basically the best way to record soft synths because you can fix/change things later on if you have made a mistake, or want something different.
 
the Piano roll view is where all this data is "Printed" / Recorded in detail.
There are (well basically) 3 ways to use Midi in Sonar. as you can see in the link below (you tube video)
https://www.youtube.com/watch?v=juIu9Rlt6v8
 
Learn to become familiar, comfortable  with the Piano roll view and you will master the craft of midi.
It's critical if you want to do certain things with soft synths. not saying you need to learn it all now, but it will let you do things unimaginable, or not even thought of before when you used the Boss to record.

prhandley
Thanks for the advice, that's exactly what I am trying to do. I'll give it a go! Cheers

check the video out I linked for you. it is basically exactly what you are trying to do.