They are humans and can cross. But, like a mortal wizard, I would say that if they cross uninvited that they would leave some or all of their powers at the door.
That's my read of it. Thresholds are an expression of the faith of mortals that they are safe in their homes and, like all true expressions of faith, are powerful and, in my game, acts as a trump if there is ever any doubt as to if they would effect something or not.
In this case, I would probably treat the threshold as a block against the hexenwolf changing form; rolled against conviction perhaps. Maybe discipline, but I would lean towards conviction.
