If you can open the site, just login as admin or host user, go in the menu to Host --> Portals select the portal you are working in and Add a new alias with the new url you call. For instance if you run the site through VS 2005 you will get and alias like localhost:6786/Website. If you want to run the site through IIS, you should add the alias localhost/SiteName ( in my example it was localhost/DotNetNuke_2 ).
In the case you can't run any site, you could go to the Database and add manually the alias. You should open the table PortalAlias or dnn_PortalAlias and add the new alias. This table only have 3 fields, the counter ( PortalAliadID), the PortalID and the HTTPAlias.
I hope it's solve your problem.
Marbelys