If they are in krivac , yes! Heat them in boiling water (in the bag) for 5 mins or longer untill hot. Make a sauce , and pour over mussels. Or add the mussels to the sauce and heat together. Any sauce you like with fish is fine , the norm is white wine ,garlic ,onion, parsleyAnd olive oil.
Also , since they are frozen , don't worry about any that are unopened they are fine.