If you can't change Nameserver (NS) records – you may park your domain:
Go to My Websites page;
Expand "Edit website" dropdown list;
Copy real IP address of your Site.pro website;
Go to domain control panel of your domain registrar to update "A" records of your domain;
Replace to a real IP address of your Site.pro website:
@  A
www  A
Note: Domain DNS renewal may start working up to 24 hours (usually it takes ~15 min). If the domain does not start working within 24 hours, please inform us by creating a support ticket.