Contrary to the answers you got so far you definately can do this in BiaB.

Press the green Seq button and select Melody track.

Click the elipsis button beside the track you want (say the bass) to move and select move to soloist track. It will ask if you want to make the soloist track multi channel. It doesn't really matter at this point so click yes.

Now go to the Notation|Event Editor and make sure you are editing the soloist track.

Using the event filters remove the patch changes and other superfluous commands such as chorus and pan.

Next use the Edit|Copy/Move Tracks command and move the Soloist to the Bass Track. Note: use move, not copy, so you don't leave midi on the soloist. The Move Tracks command will freeze the Bass track so that it won't be re-generated when you press play.

Finally set the Bass track patch using the standard radio button/drop down or GM button.

Repeat on any other tracks you want.

Now remember if you unfreeze the tracks you moved in the future you will lose the midi info to what BiaB generates based on the style so be careful.

In addition I have found the most success if you leave some tracks, usually the rhythm instruments, to BiaB. This way the song won't sound too structured if you are using it for accompaniment.

BiaB 2013 b366, RB 2013 b4, WinXP Pro SP3, Toshiba M70, 1.8GHz 2GB RAM 100GB HD. Focusrite Saffire 6 USB, Ketron SD2.BiaB Wiki