I suppose it's reasonably well-understood that Hive-engine tokens are currently centralized tokens, unfortunately, although I believe they are working to change that.

In the meantime, while I can't speak for the Hive-engine team, my guess is that once they understand that your exchange fully supports their tokens, they will unblock your account from receiving transfers.

As far as user-creatable smart-contract tokens go, there's several possible alternatives to Hive-engine tokens being designed, as I suppose you are aware. @disregardfiat is working on one and BlockTrades is also planning to create such a system. Both of these are being designed to be fully decentralized when they are introduced.