@Pipeline;
Thank you very much! It works

I used a default js plugin that converts each note into a 3-note chord.
Method using Reaper for conversion (if somebody else needs to do this someday

)
#1: Put the plugin on the track with the single-notes midi-file and then apply track-fx as new midi-take (works like freeze, so no need to re-record

)
#2: Save new midi-file and import it into BIAB as chords from midi-file.
#3: Tidy up chords.
Works like a charm

(Of course it would still be easier if BIAB could use single notes in a midi-file to create the song-progression. But it works when there are 3 notes, so that'll have to do for now)