That's your registration that's external (or is your mail there instead of at DreamHost?). It'd be easier just to use DreamHost DNS. You should be able to set the DNS servers at your .ca registrar to use ns1.dreamhost.com, etc.
Is there a reason you don't want to do this?
There may be a way to set up a CNAME at your .ca provider, but I don't know how DreamHost can be configured to respond to a CNAME.