Yes frozen crabs make good rig bait. Fresh is always better but frozen grabs will work fine. Several anglers I know regularly catch good fish on frozen prawns and shrimp purchased from the supermarket. Use plenty of bait elastic to tie the grabs on. Grabs wash out faster after being frozen so you may have to change them more often if not getting any takers. Come to think of it many of the trout and salmon taken in the Twizel Canals are taken on frozen shrimp!