No, not unless the parent domain is added to the hosting system. If you try adding a sub-domain for a domain that is not in the system you will receive the error that the original poster quotes.
As Scott said, if you have the parent domain hosted elsewhere and want to host just the sub-domain here, you need to add BOTH the parent domain and sub-domain to the DreamHost system, obtain the sub-domains IP address via the panel then add a DNS A-Record at the other host, pointing the sub-domain to this IP address.
Note; The above procedure will not affect the hosting of the parent domain. Even though it has been added to the DreamHost system, this is done purely as a 'work-around', the parent domain will still be hosted where ever it was. You will of-course have an extra web-directory for that parent domain here at DreamHost, but you can ignore that.
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