It's really a lot more involved than it appears on the surface. Legal authority, familiarity with laws and regulation, familiarity with the ports and terrain between ports, etc.
Unless you've undergone that training, a person's usefulness is limited.
I suppose you could have MPs assist in guarding detainees, engineers building detainment facilities, but that's pretty much all I can think of and the Posse Comitatus act can foul that all up in ways to horrible to imagine. You know some liberal judge will decide the involvement of the military illegal and order release of all the illegals.
C-130 gunships and Apache attack helicopters have a way of cutting through those kinds of Gordian Knots.