I do know the Nashville number system (aka Nashville notation) and even I was a little baffled

But the answer is that BIAB can do most anything you want. If an auto-generated intro is not what require then enter the chords you want and get the resulting arrangement for the intro. 1 5 1 1, 1 1 5 5, 1 4 5 1, 1 2- 5 6-, you name it and you've got it!

--Paul--