Domains registered

I setup up my server to use OnlineNic.
I have several domains already registered via Onlinenic. How can I add them to the specific users?
 

You can register only new domain in hc panel but can't transfer. Its because when you register a domain panel try to create / register in both locations ( HC panel and OnlineNic ). In that case if domain already existed in OnlineNic it will not add.