From Support
Yes! If you have already registered a domain name somewhere else, you can still use it in our system! When you create or import a site, just enter your domain name in the "Existing Domain" section.
You will then need to update the DNS for your domain name to point to the server your published site is on. Create a new CNAME record from your domain to pub1.myrdo.com. Links to instructions on how to do this for several popular registrars are listed below:
After you have created the CNAME record, it may take up to 12 hours before you can visit the site with its new address.