I don't think there's a config value for this. It does sound like a good idea though.
From a quick look at the code, if a girl explores the catacombs, the chances of her encountering a monster are equal to the best of her combat and magic skills, less one. So if she has 100% magic, she'll have an encounter 99% of the time.
That leads to her fighting 1-3 monsters with a 50% chance of them being a girl. If she wins, the girl is captured. So a construct with 100% combat stands to bring back 1.5 monster girls each foray.
Simplest thing is probably to define a % chance that defeated girls are captured (as opposed to being killed or running away) and to check that once a girl is defeated. Set it in the config file and check it in WorkExploreCatacombs.cpp
Or I'll do it when I get back, but that's not for a few months yet