I think that you have to let trade the pair ENG/BEE and let users that want to simply transfer tokens from Steem to Hive and restart the token in the new blockchain have this for free, beaucause they have already payed for this.
So:
If a user want to have a token both on Steem and Hive, is correct to pay BEE for the new token
If a user want to just transfer his token from Steem to Hive I think it's a bit tricky ask for BEE having already payed ENG.