Charging NX from Steam

So today I wanna sell my Xbox games because I hate Xbox and am giving it away. I read that you can charge NX from your Steam wallet. Just wondering how this works, and if the charged NX is prepaid or credit. I owe some people some things, and well I hope it's prepaid Dx I would just buy NX cards at GameStop but my parents hate NX and say it's a waste so I obviously can't do that. At least I can convince them to let me buy my own Steam Cards.

So how does this work? And also: I can use this NX in Maple right o-o