Naturally my Librarian plyer hit upon the boxed text in the Psychic Powers section that pointed out how useful Unnatural Willpower is for Psykers, so now he's looking for a wy to atain it. It's a trait, and it doesn't seem to be purchasable on any advance chart, is it meant for NPCs only?
The same sidebar is in the Rogue Trader psychic powers section, and Astropaths can't obtain it either. In most cases, Unnatural Characteristics (of any kind) are for NPCs; I'd be reluctant to allow anyone access to (additional, in this case) Unnatural Characteristic talents without an extremely good reason.
Many Ascended careers get unnatural traits at rank 9 (which is 1 rank above the final rank in the DH core rulebook and at 13,000 xp). So except for that level of characters then it should probably be disallowed.
FYI, rank 14 (35,000 xp) Primaris Psykers get unnatural willpowerx2. Also, Inquisitors get it at rank 9 (13,000 xp), but they usually go through at least a couple ranks of another career between the DH core rulebook and their rank 9. So, if the characters they are playing are of that level, then it could be allowed, but you should look at the psychic powers to see the effects it will cause for that character. A blurp from the ascension book:
In Ascension, it is possible for psykers to gain the
Unnatural Willpower Trait. The GM should be aware
of the impact of Unnatural Willpower on psychic
powers. Often, it is very high-rated Psykers who
possess Unnatural Willpower, meaning that characters
with this Trait will be considered both extremely
powerful and extremely dangerous by their peers.
So with that info in mind, you as a GM would need to decide if it works in the game you are running.