Is it possible to record the metronome track in a mixdown, to build up backing tracks?
Only by routing it out of a separate output on your sound device, and looping it back to a spare input. If you don't have a multi-input and output sound device, then I'm afraid you can't do it directly, no. It may be possible that if you use something like Virtual Audio Cable (assuming you're on a PC not a Mac), you might be able to tap into the metronome output feed and have it presented as an altenative input, but I've not tried that - it's an almighty pain to set up, whatever you do with it!
Easy to do in Windows with VB-Audio Virtual Cable. In Audio Hardware, change your Default Output to "CABLE Input." In the track where you'd like to record the metronome, change the input to "CABLE Output."
Thanks I will try it!1