use subdomain as primary with domain mapping?

Hello,

I would like to setup domain mapping to forward requests to cuartetotanguero.com to cuartetotanguero.moretango.org (the subdomain in the the WP Multi-site install.) To clarify, the user should see the domain cuartetotanguero.moretango.org after entering the domain name.

Is this possible?

Also, I can't add the domain name because it doesn't point to my current ip address. I need to add it before changing the dns settings to make sure everything continues to work properly until tested. So how can I install a domain before I update the DNS?

Thanks,
Ben