Originally Posted by juleska
Could it be that warging is a skill and not merely a gift? While certain people are born with the ability to be a warg, there are some who can learn the craft? If so that would make sense that the Faceless Men would have warging in their arsenal.
"One man in a thousand is borne a skinchanger, and one skin changer in a thousand could be a greenseer."
That's what it says on the Westeros wiki. That may be factually wrong, but it sounds like something that was mentioned in the books, though no reference is given. Even if it was mentioned, it doesn't necessarily mean that you can only be born with the ability, but I don't think it's something that can be learned/taught, if only for the fact that it loses its distinction of being special or rare.