I'm not sure I got it right, but I think you're wanting to host a website using your Dreamhost account and using a domain name that you've already registered somewhere else. That should work just fine.
I recommend that you e-mail support for this. Better yet, use the support page in the panel and ask that DH does it for you -- that way you're sure that it gets done right, just in case you're worried that you might mess it up if you do it yourself through the panel.
Dreamhost support will know which plan you have and if your plan has "room" left for the domain then they will use that instead of billing you of course.