There are a few harmonies that are designed with no melody. In my system, for instance, harmonies 51 and 52 are "ThreeP -NoMel" and there may be others. Using that on your melody track will create a 3 part harmony from your melody but won't delete it.

If you want, you can then convert the harmony to the soloist track. Since the harmony has no melody, you just get the harmony notes. Now set the melody harmony back to zero (no harmnony) and play on. I had some trouble assigning midi instruments but I am sure it is just something I am not doing correctly.

So you can put a melody harmony on the soloist track. If you change the melody line, don't forget to repeat the harmonization as the melody and the harmony / soloist are no longer linked

BIAB for Windows Version 2019 (643)
Windows 10 (1909)