BUT -- it's not very compatible with inherited power/weakness combinations.
Of course, they might be mistaken about whether they were undead. There are bean sidhe (banshees) that are said to be ghosts of murdered women, though the very name means merely "fairy woman." And the Cold Lad of Hylton was said to be a stableboy's ghosts, despite being obviously a less than helpful brownie (of which there were many non-ghostly ones in folklore).
Or perhaps they need the corpses (or the appearances, for ghosts) of the recently dead to work. (That's a real weakness.)
Or perhaps it's hereditary with the additional weakness that it only works after you would have died, had you been normal.
Or maybe I just omit them all.