Yes, we do have rules for opossums as Critters!
You can also easily adapt any animal into a Critter by selecting its type (Fuzzy, Feathery, Scaly, Warted, Shelled, or Crawly) and then mixing and matching the ~200 animal talents (Critter powers) in the full rulebook. Every Critter gets 2 animal talents.
In terms of adapting Blister Critters to a pre-/non-apocalyptic setting, that's also easy: just describe the world differently! Perhaps humans exist, and your Critters are simply living apart from them. Perhaps the Critters are escaped test subjects from a secret lab, which is why they are intelligent and have powers…
The one thing to keep in mind is, in our canonical setting, there are no humans, so we don't have rules for them, only animals. But you can just give them high scores and assume they're generally in charge, pre-Poof (pre-apocalypse), and everything else works.