Forgive me if what I’m about to say is obvious or if you already know it, because I read in your other thread that you didn’t know much about musical theory, chords, etc.
Flat b note = Sharp # note
Bb = A#
Eb = D#
Ab = G#
Db = C#
Gb = F#
These notes are the same, or in other words they are “enharmonically equivalent”
So, technically speaking Renoise can obviously handle the notes, but it can currently only represent/display them as sharps. You will have to adjust your script to take this into account when generating the note data, but your script’s custom GUI can obviously display them in any way you want to. You could still let the user visually choose from chords with flat notes, for example.