My old Yamaha keyboard does support user Style files, but only in midi, not audio unfortunately.

Ableton software, Melondyne etc, have the ability to convert an audio file into a midi file interpretation. That kind of function in BIAB would be perfect. I could then compose a small 16 bar arrangement using realtracks for example, convert it to midi, then process it into a Yamaha Style file to use as an onboard user style on the keyboard.

You can convert a monophonic audio track to a MIDI track in both BiaB and RB:

https://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=481127

{edit} I have not used this conversion part of BiaB in a long time. I have found Melodyne to be the best audio to MIDI converter. YMMV

{edit #2} Don't forget that many RTs come with MIDI charts already.

It will save to midi, you can save the RealCharts as an option.

Yes, BiaB has a monophonic audio to midi converter as noted above.