This is referred to as a reseller account. Please log a support request advising you would like a reseller account, and also list the primary account or domain you wish to use. We will then assign the others to this primary account. When you next login or refresh the page you will see a drop-down at the top listing your accounts.
You may still be required to log in to your original accounts if you wish to use features that utilise single-sign-on, such as phpMyAdmin.
