Glue, and loose hair.
Beavis and Butthead had to relocate hair from their heads to make their beards. Your werewolves have a modicum of hair at all times. They are very careful to keep shed hair clean and dry. Then when needed, they roll in sticky stuff (which they also have on hand) and then apply the reserve fur. Presto: Beards! Or Fur! Both!
You could have the sticky stuff be spray-on if you wanted.
I was sure I would find an image of one of those hairless cats, with a bunch of spare hair that the owners had applied. I understand they are naturally sticky, those cats. But no luck. Anyone who can find such an image please add it. Anyone who has such a cat, please produce such an image and add it.