A single DNN Portal can have any number of domains pointing to it. For example a client I had had 30+ domains all related to the site pointing to the one DNN portal, needless to say the site ranked well!!
All you need to do is login as host go to the site settings page and look at the bottom of the page. You will see a list (only one entry to start with) of portal aliases that can be used to access your site. Just add whatever other domains you want here.
Philip Beadle (MVP, MCAD,
DotNetNuke Core Team)