I had a blood slave convoy unexpectedly attacked recently, and yeah they chipped in and one kicked someone's behind. Mainly because that enemy was distracted by a Vine Ogre...
She didn't turn into a unit though. I think maybe they have to get damaged before they turn into units? I didn't lose my commander, though.
A more probable explanation is that girls left without a chaparone for a month in a foreign land may have a good chance of er, enlisting. Or something. I dunno. It'd be nice to know the rules and the reasons for them.
PvK