Can I make salt cod from frozen cod?
You should have no problem using good quality frozen cod for salt cod. Freezing breaks up some of the tissues and changes the texture, notably making it drier as freezing drives out some of its moisture, so you might not need as much salt to dry it to the same level. 

In some places the "fresh" fish you see in the store has actually been frozen for the trip before being thawed and sold for more than the same fish still frozen, so whether you'd be getting fresh cod is debatable depending on your location.