As the reseller, you can add an alias to an user on behalf of your customer.
4. Now that you are logged in as the customer, navigate to the User List under My users.
5. From the User List, choose the mailbox you need to assign another user access rights to and click on Edit User.
6. Under the Microsoft Exchange service tab, select Manage Aliases. The Manage Aliases column should contain a list of available aliases with one set as the primary. To add a new alias simply add in the desired alias and press Add Alias. The alias will now appear in the list. Always Save Changes.