This is quite a technical question, I think.
In my opinion there has to be a way (prism.exchange does something that might be similar and so does any decentralized exchange) to manage other blockchains wallets like this:
I have a, let's say, Dash Wallet. And that Dash Wallet is controlled by nothing but an Ethereum Smart Contract. So only the smart contract controls the private key of said Dash wallet.
In my opinion there are two difficulties to overcome here:
1) How do I manage wallets on other blockchains within a smart contract? If I understood correctly network traffic is not possible within smart contracts. It could, however, sign a transaction and a service controlled by anyone could deploy the transaction to the correct blockchain.
2) How is it possible to manage another blockchains wallet without giving away the private key to anyone who audits the smart contracts code?
I hope I described my questions sufficiently, if not please go ahead and ask
