I'm sure this thread is pretty much dead by now but I thought I would toss in my 2 cents anyways.
This is going to sound confusing because I'm not always incredible at explaining things succinctly, but as I said this process is well refined and it's nice and seamless for your client(s)
The method I've come to through many refinements for external domains being transfered into dreamhost/wordpress is this:
create the orginal/final domain in dreamhost domain management. For example client x wants x.com moved into dreamhost and (optionally converted to a wordpress site), so I set up x.com in dreamhost
set up a mirror as a temporary subdomain on another domain that you currently have set up in dreamhost. (let's call it y.com) so you would set up a mirror at x.y.com as a mirror of x.com
one click install wordpress at x.com, import your content or copy the old wordpress install over into dreamhost.
Make changes/updates until customer is satisfied with the new version of the site
update dns records so that x.com is pointed to the dreamhost version. (I usually just get people to change the NS records to dreamhost)
The only tricky part is there are a few rows in the database that need to be changed around a couple of times. You can do this with relative ease in phpmysqladmin which you can get to through the dreamhost admin panel.