I haven't actually tried it, but one work-around that comes to mind is to use the "Realize Chord Symbols" plugin in Sibelius. That would use the chords symbols in your Sib file to generate a new track with your symbols translated into the appropriate notes. Import that track into BIAB and have BIAB translate it back to chord symbols. As said, I haven't tried it, but I bet it would work quite well.

HTH

Philip