When I registered a .co.uk domain once I added it in manage domains it showed up straight away but took a while to actually become active.
Basically if you added it under manage domains it should show up straight away even if the domain isn't with dreamhost.
When you click add new domain/subdomain and enter the details it should then add it to the list straight away. (Just tried and even putting in a fake domain shows straight away)
If this isn't happening then it could be a problem you would need to contact support for.
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