There isn't a forum for Costume Quest so I'll ask here.

I played the original and don't want to replay it. If I buy it here with DLC, can I play just the DLC?

Hi there!

If I am correct, the DLC is the one with the something on ice. I haven't played costume quest in a while, but the two are separate even if they are the same. In other words, when you start up the game you will be able to choose which of the two you want to play. In addition, you don't need to have previously completed or replayed the original in order to play the new storyline in the DLC.

So yes, you will be able to play it without replaying :) I hope that helped.

Yes, there is one:

http://www.gog.com/forum/costume_quest

Expect a huge cliffhanger when you finish the DLC, presumably to be resolved in the forthcoming sequel.