This is probably closest to what you're looking for:
Arkel Bug
Some people have posted here to stay away from the
Nashbar Action Pannier because it's not a real backpack. The entire black panel visible in the photo is a dry bag that can be removed.
My solution will be (for now) to buy a set of Ortlieb Back Packer Plus panniers and stuff my North Face backpack in one of them. If the backpack isn't overstuffed it should fit into one of those panniers without too much convincing. And if I need to carry more I can just fill the other pannier.