So you basically want to host the domain under the other DreamHost account?
In situations like this, but with the domain registered externally to DreamHost, the normal procedure is to change the name server configuration for the domain to point to the DreamHost DNS servers then add the domain as fully hosted in the DreamHost account that will be hosting the domain, in your case the account that belongs to 'someone else'.
Since the domain is registered here at DreamHost, the first step (changing the DNS records) may not be required. However, you will still need to add the domain to the DreamHost hosting system under the account that will host the domain.
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