Originally Posted by mtzGr
"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.
I am pretty sure that quote may not be exact but certainly sums up info given to Bran, I think after they get to the cave and into the underworld. Of course could just be my brain expelling posterior breezes also.