This is the part of your post that I find confuses me to the point of not being certain what you are trying to do.
Domain: An SLD that you have to purchase a registration for. An example is yourdomain.tld (example.org for the example challenged).
Another example is example.co.uk
Subdomain: A "domain" that you create as a child of a domain that you register. Even though this is technically a domain, you will confuse a lot of people if you refer to it as a domain. I always try to refer to these as subdomains.
An example is blog.yourdomain.tld
Another example is blog.example.co.uk
Again, the whole country code TLDs make things confusing for everyone. Theoretically and pedantically, example.co.uk is a subdomain of co.uk, but no one in their right mind would say this with any hope of anyone understanding what they are trying to do.
So with that in mind, what domain are you trying to host where? Use examples!
But, let me use my psychic powers and guess that you have two domains you want to host. One domain is registered here. One domain is registered at another company.
To host the domain you registered at another company here at dreamhost, all you need to do is:
1. At your other registrar, set your domain's nameservers to ns1.dreamhost.com, ns2.dreamhost.com
2. In your dreamhost panel under "Domains>Manage Domains", "Add New Domain / Sub-Domain".
3. Wait for DNS to propagate.
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