If you have a domain in your Billing Centre account that is not already being used on a hosting plan, you can easily add email and hosting services to it.
Once logged in you see the main account summary page.
Follow the menu option to hosting and select the plan you want to order.
Once a plan has been selected you will be asked to confirm a domain name to allocate to that plan. If you are already logged in at this stage the system will check your account to see if you have any unallocated domains that can be used.
If you do then you will see the following option ...
"Select a domain from my ACCOUNT"
If you select this option then a drop-down list of unallocated domains will appear and you can choose one to assign to your new hosting plan.
From there you can continue and complete the order in the normal way.