No, I am just a regular customer. As ardco said; the best way to obtain an official response is to lodge a support request.
Well, you aren't 'registering' the domain by doing this, you are just adding it to the DreamHost hosting system. As I understand it, nothing you do at DreamHost can affect the DNS records for the parent domain, as long as you leave the name servers for the parent domain as they are at the registrar and don't point them to the DreamHost name servers.
Firstly, let me confirm exactly what it is you are trying to do. If I understand you correctly, you have "otherdomainname.com" registered and hosted elsewhere, you wish to host "blog.otherdomainname.com" here at DreamHost and have it mirror the content at "domainhostedwithDH.com", but you do not want to point directly to "domainhostedwithDH.com", using a CNAME for example, because of SEO considerations? (*whew, that was a mouthful ).
If I have that correct, then this is how I would do it...
1...Add "otherdomainname.com" to the DreamHost system via Domains -> Manage Domains -> Add New Domain / Sub-domain in the panel. Once again, this will not affect the hosting of the parent domain elsewhere, this step is only required so we can add the sub-domain without error.
2...Add "blog.otherdomainname.com" to the DreamHost system via the panel and set it to mirror "domainhostedwithDH.com"
3...Obtain the IP address assigned to "blog.otherdomainname.com" by going to Domains -> Manage Domains and clicking the small [DNS] link next to the sub-domain. The IP address can be read from the main A-Record for the sub-domain.
4...Take the IP address obtained from step 3 and use it to create an A-Record for the sub-domain at the parent domains registrar.
Note1: The IP address assigned to the sub-domain can change at any time as DreamHost moves things around, you will need to keep on top of this and modify the A-Record as required.
Note2: I have not personally tried the above procedure, but I don't see why it wouldn't work.
Once again, I am only a customer here, for an official response submit a support request via the panel.
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