I'm sure there are many approaches you can take to including Backup Guns in a match.

But as IDPA rules specify a maximum of 5 rounds for a BUG, it's likely that you'd either modify the requirements for some of your stages (i.e., only the first X number of targets) or set up separate stages for BUG shooters.

Either way, you may well have a different number of total stages for BUG, and possibly a different number of strings on stages.  So the software needs to be able to know when a shooter's BUG data is complete.

You could, of course, define a separate BUG match, and just set it to the number of BUG stages.

NOTE:  this option requires that Use advanced menus be checked.