If you use an arranger as a controller for your software, you can assign like lead #1 or any other and have the arranger play it's part providing you have the midi set up right.

As far as the beat being derived from your software, an arranger, many times, has this capability but I have never done it.

Another way is triggering rhythm tracks in the computer from the keyboard if you don't use full accomp.

Many know much more than I however.

Bernie