Here are the steps for setting up a Domain Alias. A Domain Alias will allow domains to load/show the same site as existing domains, without changing the domain in the visitors URL/Address Bar.
To create a Domain Alias, do the following:
1) Log into the Domains section of your hosting account.
2) Select the Domain you would like to set the Domain Alias on.
3) Under Domain Aliases, click on the Add Domain Alias button.
4) Add the domain name and click 'Create DNS zone', also choose 'New Mail Domain Alias' if you want to use the Mail Domain Alias feature.
5) Click Submit to create the Domain Alias 6) Update the Name Servers on the new Domain Alias to match the Name Servers listed in the Domains section of your hosting account.