Oh dear, sorry to hear about this mate. But I really dont think Cryptopia can do anything to return you the whitecoins, seems like it is out of their scope this time. I know this feeling suck, perhaps take this as a lesson and move on.
also, most of the transaction on the cryptocurrency space is irrevocable, once you are going to make a transaction, make sure first that your decision is final, also to consider if there will be a factor that can affect the system when you are going to make a transaction. Because in that case, I assure you, you'r fund cannot be recover.