You can log into each account under the battle.net account seperately just as you normally do. Maximizer will still work, however the input of the username won't be needed since all accounts will use the same name, you will have to cycle through and select which account you're logging into.

Here is a thread I posted when I did my conversions, lots of good info in the replies.

Battle.net Accounts and you: How I merged 7 wow accounts to 1 login.