I don't know about now, but pre-The New 52, all of Darkseid's forces were culled from Apokolips; Earth was supposed to be off-limits to both Apokolips and New Genesis by dint of a treaty (the one where Highfather and Darkseid exchanged their infant sons) between the two worlds, a treaty Darkseid was beginning to violate when he and the whole Fourth World crew were first introduced. Oh, and Granny trained, or oversaw the training of, ALL of Darkseid's forces, not just the parademons and Female Furies (she also sees to the training of his regular troops, as well as special types like Glorious Godfrey, Kanto, and so forth).
This is not to say that you couldn't come up with a good idea to explain Darkseid's impressing a human (I'm assuming a super powered human, otherwise I can't imagine him/her lasting long on Apokolips, unless they're Batman-esque or something). He's always been one to figure out ways to get what he wants.
Exactly. Darkseid's always been one to use Exact Words, Loophole Abuse, Rules Lawyering, and then just flat out cheating when he really wanted something. So if he felt he could get away with snatching a few humans, why wouldn't he?

So these guys are the shock troops of Apokalips, taken from the hunger dogs, the poor of apokalips(or bred in labs apparently).
Granny Goodness trains young children to be Female furies(and has at times spirited away children from other planets as well).
So this makes me wonder if sometimes, Granny takes earth children, and those not worthy of being Furies are turned into Parademons.
I ask because of an idea I've had that I want to try, and this is something I need to know.
One Strip! One Strip!