Yes King was a bit vague about how/why they became True Knot. Maybe they'll appear in another book?
He only said that they had always existed. They did target some people with special abilities- the girl found putting men to sleep to rob them was one they absorbed into their tribe. But they needed steam once they were initiated to stop them from ageing and dying. Steam kept them young and came from the dying breath of civilians with the Shining. I think the True Knot used this as 'food' allowing them to maintain their abilities. I don't think Steam gave them special powers other than eternal youth.
I think the True Knot and steam are like Vampires and blood.
What they consume keeps them young but doesn't give them powers. Andi had her powers already and was somehow converted to become True. Rose hints that the same happened to her many years before in Ireland.
There must have been some original True to turn the others but that wasn't explained and I would have liked to know who was the first. Perhaps that's another book.