If you go to the Verizon Site-Builder page and then click FAQ, you read this for #13:
13. Can I use my own domain name? Answer: If you own a domain name and would like to use it as the main address of your site, you will need to make arrangements with your domain name provider to redirect visitors to your Verizon Online mysite.verizon.net address from your current domain name.
Verizon Online does not currently offer personalized domain name registration, but we are working on providing this option to you in the near future. Updates will be provided on the Site Builder home page, so check back often.
The above statement has been listed for quite some time. Does anyone know whether this option is now available? (There's no updated statement about it on the Site Builder home page, and there is no one to ask!)
I don't think it has been enabled, if you don't see any options for parking your domain in your Account settings or PWS Settings. So if it's been there for a while, it's been forgotten most likely.