You can have up to 10 additional domains in one Premium package.
To order one or more domains, go to the menu on the left-hand side and click "My package" => "Domains".
Click on "Order additional domains" and perform the order.
After the domain has been successfully set up, your website is now reachable via your new domain (and also with your first domain).
Just click under "Primary domain", where one of your domains should be your primary domain. The primary domain is the one displayed in the address bar of the visitor's browser.
If you want to transfer your domain to another service provider, please check this page: How can I transfer my top level domain from hPage to another provider?