I have only done this a few times and it goes back to how i used to add extra midi tracks to a powertracks seq file after exporting from BIAB.

create the song in BIAB and save it. export as a midi file, open in PT and save as a seq file. close it.

what i used to do in PT was go back to the BIAB original and audition several styles (which in BIAB is easy and fast and even allows you to solo the instrument), find one with the new or extra instrument i liked and export to clipboard as a midi file then close the BIAB original without saving to preserve the original arrangement. open in PT from the clipboard, copy the track i wanted, close the midi file from the clipboard without saving and then open the original PT seq file and paste the new part in.

i think the same process would work in RB but i haven't tried it. and the original BIAB file would open without the export to clipboard i used to have to do with PT although i'd still use the midi clipboard option for the new part as I wouldn't want a raft of saved redundant files. i see BIAB and RB as steps that work together. in this case i'd go back to BIAB rather than work in RB.

once you have the midi info you can still change patches - HiQ strings for GM strings etc but its a way of getting new midi tracks into a seq file which i find less clunky by auditioning in BIAB rather than guessing in RB.