By default, ForScore will only let you register a shooter once in each division in a given match.  If you want to let shooters register more than once in a division. mark the Allow Reentry checkbox in Program Preferences.

Having done that, when you attempt to register a shooter a second time in a particular division the system will warn you but permit it:

 

 

On the main score entry screen, Reentry listings are flagged with a yellow highlight:

 

 

And on the shooter entry screens:

 

 

And results.  In this example, the reentry places amongst the other shooters.  You can also specify in Program Preferences that reentry shooters be listed at the bottom of each division/classification.