They are releasing them with an addition to those who don't have the L&K DLC will be able to see hte Revolver character - so my guess is they will add in the L&C DLC characters to that as well - so should be out soon!
I doubt they'd be retroactive just based on the way the challenges in the game work.
For example, if you're doing the hunting challenges that are already in the game you may have killed a million rabbits but it isn't going to count them towards the challenge until you unlock the challenge.
That is a good point. I didn't think of that. I love the game but I think my hunting days are over. Scarred by beavers and bears and cougars. I didn't even want to do the kill 2 with explosive rifle in the new DLC.