No, if you already have a domain name, you can simply type it in when making an order and the system will ask you whether you wish to transfer the domain name to Stablepoint or update the nameservers to 'link' your existing domain to your new hosting service with us.
If you don't already have a domain name, you can purchase one when you order your hosting through Stablepoint, or you can purchase one from another provider and point the nameservers on that.