As reported on Kotaku, Valve has anounced a forthcoming expansion named The Sacrifice which will allow players to team  both the casts from the original game and sequal.  Best yet the DLC will be compatible with both version of the game.

"The Sacrifice tells the story of the last Left 4 Dead 2 expansion, The Passing, from the perspective of the original game's cast. You'll find out how they managed to meet up with Left 4 Dead 2's cast, and at the end, will be able to choose for yourself which of the original cast dies (hence the expansion's name), rather than Valve simply pencilling it in."

The expansion will be available for free from Steam on PC and Mac.  For you Xbox players, sorry you are going to have to pay for it.