To expand on what Robert is saying, you'll be able to host your .ca domain here at Dreamhost. What you'll do is to set the nameservers of your .ca domain at your registrar to ns1.dreamhost.com, ns2.dreamhost.com, and ns3.dreamhost.com. You'll then "add hosting" for your domain here at Dreamhost through the Dreamhost Web Panel. At that point, you'll be all set and able to do what you want with the site - upload files, install web applications, create subdomains, etc. You will still need to either keep your existing registrar or find a registration-only registrar for your .ca domain.
Given that, you'll still have a free lifetime domain registration available through your Dreamhost web hosting account (free for the lifetime of your hosting account). You'll be able to register (and host!) any other .com, .org, .net, or .info domain using that free registration. And don't forget that you can host as many domains and subdomains as you want here.
(Apologies for seeming a bit like a salesperson, but I did want to eliminate as many possible areas of confusion for you as possible)
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They're Dreamhost coupons!