Hi jrlamkin2,
Welcome to the forums.
Are we talking about BIAB 2018? If so, read on.
Firstly a bit of BIAB background.
The Bass, Drums, Piano, Guitar and Strings tracks are reserved by BIAB to create backing tracks. The Melody and Soloist tracks are set aside for user note entry.
The above being said, it's possible to modify notation on the Bass, Drums, Piano, Guitar and Strings tracks and then use the 'Freeze' command to keep the notation intact and to stop BIAB regenerating the track next time the song is opened.
Also, BIAB only loads midi file onto the Melody or Soloist track.
With the above in mind, if you want to write a bass line in Finale... save it as a midi and then import it into BIAB's Melody. From there, it's possible to use "Edit | Copy Special | Copy/Move Tracks" to transfer what's on the Melody track to the Bass track. Once that's done, the Bass track needs to be frozen.
Lastly, if you are writing your own bass line, it will only play if you use a midi bass on the Bass track. Realtracks are not affected by notation because they are audio recordings.
Regards,
Noel