I don't doubt that in the PItW universe a partial shift is theoretically possible; since if you try anything long enough, you'll either succeed or die of old age.
Kris points out the Werewolves themselves don't really know how they do what they do...other than some vague biological or religious potentials...and even then, they don't agree which is which. If you don't know how you do something, it's kind of hard to control it.
Besides; think! The Wuff with Sarah's head--or vices-versa? Jean, cute as she is, would look...odd with half her coat in. Sort'a hairy, y'know. If you mean shifting parts of the body while the rest remain either Wolf or Human; that would just be strange.
Kris's all or nothing approach seems the best at this point, with one possible exception.
Gus, as a wolf, minus the fur--that would look just about appropriate for the guy. Ugly as all get out, with a ratlike tail.
Sorta like Underworld...