As of the moment there is a bunch of online wallets that can do that like Coinbase but it is only like less than 5 cryptocurrencies is in their service, but if you really want only an address that you can change bitcoins to altcoins and vice versa then you can use exchanges for that because you can trade there in an instant. but you can't hold the private keys when you are in exchanges so creating a local/online bitcoin wallet will help to secure your coins.