Bill:

If you're not using the Melody Track or the Soloist Track, you can just copy the notes you entered onto the Guitar Track (using jford's method), to either of those tracks - just "pretend" it's a "new" bass track and set a bass intrument for it.

Then go to the first of those 20 measures, and using F5, mute the bass part (unless you want YOUR bass part to play along with the BIAB part), and then at the end of the 20 measures, "reinstate" the BIAB bass part, again using F5.

Good luck!
LLOYD S