You can have up to 10 additional domains in one Premium account.
In order to order one or more additional domains, please click in the menu on "My package" => "Domains".
Click on "Order additional domains" and perform the order.
After the domain has been successfully setup, your website is additionally reachable via your new domain (and also with your first domain).
Just click under "Primary domain", which one of your domains should be your primary domain, which means it is displayed in the address bar of the visitor's browser.
If you want to transfer your domain to another service provider, you can just create the required "AuthInfo" by clicking on the appropriate button.