Record the Metronome Track into a Mixdown

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."

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!