When you create a new account in your AD , make sure that the username is as follow for example :
john@yourdomain.com
and make sure to put john@yourdomain.com in the email specific field in his AD account.
Once your synchronized , you should be see the yourdomain.onmicrosoft.com as secondary.
In case not , there is a way to do it from powershell , but I am not sure if it is still applicable after upgrade.