This is a society with serfs that, for example, worked on many of our projects for no pay in the beginning before we even had revenue. Let's be realistic: there's a kind of slavery at work in this society, and I see no reason not to put these people in that system.
Hm. This does bring up the question of what stabby meant by "slavery".
That's getting ahead of ourselves. First things first, this is not an "Aw mom, these barbaric foreigners are so adorable, can we keep them?" "Only if you feed and water them yourselves, dear" situation. We are not playing tamagotchi with barbarians that in other places are committing genocide on our people. First, we use them for intel, then we try to use them for hostages, then maybe we put them in our mines or fields. Call that what you will.
I'm imagining the latter, with decent working conditions and the hope that they become acclimated enough to our culture that they or their children can become full citizens.