If you set in Preferences to Allow multiple instances of Biab
start another instance and open a copy of the SGU,
generate the multiriffs,
in the Biab with your song, Audio menu > Audio Chord Wizard (Audio Edit),
in the multiriff instance drag the track you want from the DROP WAV into the Audio Edit of the song instance and set it to the bar you want it to start on,
then Audio menu > Move Audio to Performance Track..
do the same with the midi realchart DROP MID it into the piano roll melody track then move it to another track or cut n paste.

more info
https://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=583882