If I understand you question correctly, you should not incurr any additional costs to have your domain pointed to the Dreamhost DNS servers. This is accomplished on your *registrar’s" site.
If should elect to transfer your domain registration to Dreamhost (which you are not required to do to host your sites at Dreamhost) the transfer will cost $9.95 paid to Dreamhost (hopefully your current registrar has no hidden “transfer fees”!), and will extend your registration for a year from it’s current expiration date. Dreamhost actually gives you one domain registratioon “free” when you sign up for hosting, so you could actually save some money.