I read the first... oh, maybe four books in that series. I'd say the setting/powers/creatures from first few books would be pretty easy to convert. The last book I read got pretty freaky and had strayed pretty far from the original premise. So I guess there's also the question of what part are you wanting to convert.
A campaign based on characters and organization in the first pair of books would rock, and I don't think it would be too difficult to do. To be honest though, it's been close to more than ten years since I read those books, so I'm probably a bit shady on the details. Forgive the plug, but Green's Guide to Ghosts may be of some help on the ghostly side of things. As far as powers are concerned, I'm afraid I can't be of much help there. My gaming library is pretty narrow in scope.