Cross chain atomic swap on bitshares DEX,is it possible?

As i know bitahares have on chain atomic swap
I wonder if it is possible to make it cross blockchains as follows
Creat an assest for the tokens to be exchanged linked to multi signature address for that same token one of which with the user and the other private key saved online so that the user cant withdraw the money as long as he have open orders
Once the user make an order to sell his token he enter his private key and once the offer is accepted the bot will automatically approve the transaction with the second private key

It will ofcourse have more cost as it will take the bitshares fees plus the other coins miner fees yet it will grow the market cap of bitshares and help the marketing campaign as truley decentralized and trustless exchange
Hope i can found such a feature in my favourite exchange soon - if it was possible