This article describes how to add UPN suffixes to a forest. Adding these suffixes gives you the ability to use a friendly user-logon name that does not match the domain's or parent domains' naming structure.
Adding a UPN Suffix to a Forest
Open Active Directory Domains and Trusts.
Right-click Active Directory Domains and Trusts in the Tree window pane, and then click Properties.
On the UPN Suffixes tab, type the new UPN suffix that you would like to add to the forrest.
Click Add, and then click OK.
Now when you add users to the forest, you can select the new UPN suffix to complete the user's logon name.
For additional information about UPNs and user logon names, click the article number below to view the article in the Microsoft Knowledge Base:
243280 Users Can Log in Using User Name or User Principal Name