I have already registered a domain - can I use it in my NuBlue account

This is not a problem.  You can create web space for existing domain names, or even order web space for existing domain names if you have not purchased hosting yet.  Once you have created the web space you will need to change the name servers so that your domain name points to our servers (further details on this process can be found under the ?How do I change name servers?? section of this Knowledgebase.