You add a server to a domain just like you add a PC.
Creating a new domain for a single server usually isn't recommended. Besides, you can't add a domain if the server isn't a DNS. On the other hand, the server doesn't necessarily have to be in a domain, but it's easier to control access if it is.
I have one server running dns I would be adding a server just to do faxes but I would put it on the domain so that anyone that has access to the domain can fax out of it. is this not the best way to go about it?