I haven't tried this, but you might be able to add the parent domain to the DreamHost system, even though you don't own it, it just won't do much. However, doing this may allow you to then add the sub-domain to the DreamHost system.
Of-course, you still have to figure out how to configure the DNS for the sub-domain to the correct IP address for your particular DreamHost server.
Assuming you do get all this working, it could still be problematic if DreamHost changes the IP address of the server you are on.
It would be much easier to just create a free dreamhosters.com sub-domain, or pay the $9.95 a year for a 'real' domain.
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