A quick search pulled up this little article:
http://www.crustaforum.com/board/showthread.php?377-Crayfish-Plague which says:
The spores do sit on everything within the tank, however, they register even before forming a cyst whether the potential host is really a crayfish. [Shrimp] definitely carry the illness!
Since it says that the spores of the disease can sit on everything, I would advise you not to add ANYTHING from a native stream or anything of that sort. I'm sure the snails would be great food for your critters, but it's simply not worth the risk in this case.
However, you can purchase snails from someone/somewhere that you know have not been housed in a tank or same environment as North American crays and I would think that's okay. But it is very good of you to ask these types of questions BEFORE adding things to your tank rather than later. You're a good crawdaddy!