If you want to use an exisitng domain of yours as part of your RFE Hosting order rather than register a new one you simply need to select "I will update my nameservers on an existing domain Or I will register a new domain." on the domain page when placing your order and enter your domain. Then process your order as normal and once you have received your confirmation email you can point your nameservers to RFE Hosting based on the server you are assigned to.

The changing of nameservers may take up to 48 hours.