I just got a krampus sack by unwrapping one of bundled supplies from loot stash. I assume that as krampus sack got somehow wrapped in the first place, it should be possible to wrap it back. As krampus sack is just a variation of backpacks, and a quite big one, other backpacks should also be wrappable.
1. Be lucky enough to get krampus sack from one of loot stash's bundled supplies. 2. Wonder how the sack managed to get wrapped in the first place, as backpacks can't be put inside by players at the moment. 3. Come to Klei forums to report this as a bug.