I am not sure this is the ‘right’ way, but this is the way I would do it
Firstly, you can add the domains marnik.org and blog.marnik.org to the DreamHost system without affecting your existing sites. Just add them as ‘fully hosted’ via the panel at Domains -> Manage Domains -> Add New Domain / Sub-Domain. Once again, this will not affect your existing sites, as long as you do not configure the DNS for marnik.org to point to the DreamHost DNS servers.
The above step will result in two directories in your home directory corresponding to the domain and sub-domain. You can now upload your site files, create any required data-bases and basically get things working.
To access these domains for testing purposes you can either modify the hosts file on your computer, so the domain names resolve to the IP address of the DreamHost server they are located on, or you can create free dreamhosters.com sub-domains and set them to mirror the real domains. More information on this second technique can be found in the wiki article linked below;
Once everything is working as you like, you can configure the domain DNS to point at the DreamHost DNS servers and (if you want) transfer the domain registration to DreamHost.
I hope the above makes sense. If not, ask some more questions and I will do my best to help.
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