The lore is already there. Werebears are supposed to be a major nuisance in Skyrim. There's really nothing in game aside from a few books talking about certain individuals being unable to find them and a joke by M'aiq that addresses their absence in the first place, so I really don't think they need to come up with a lore reason. They'd just be correcting a past mistake/oversight/whatever you want to call it.
I would hope that they just stay with the infected by a werebear lore than, and make it traditional forced transformations and such.

That sounds like one of my characters.