How to know if frozen clams are good when cooked
Per

http://homecooking.about.com/od/foodstorage/a/clamstorage.htm


  Do not freeze clams in their shells. To freeze clams, shuck them,
  being careful to save their liquid. Rinse with salt water (1
  tablespoon of salt to 1 quart of water), and place in a container with
  the reserved clam liquid and additional salt water so they are
  completely covered. Frozen clams will last up to 3 months at 0 degrees
  F. Thaw frozen clams in the refrigerator before using, and never
  refreeze them.


While About.com is not always the best, in this case their information passes the gut test.  Frozen clams certainly would be killed, and so wouldn't open upon steaming.

The thing about never refreeze is almost certainly a palatability issue, not a safety issue, as long as they stay in the safe zone (under 40 degrees F) and are not unfrozen for very long--but clams are highly perishable. I would exercise considerable safety.

Sadly, I found conflicting information saying freezing them in the shell is okay--but they didn't offer any detail at all, so I its hard to give credence to:

http://nchfp.uga.edu/how/freeze/clams.html

despite their being a university sponsored site.

Being conservative, I would cook then freeze.