As the reseller, you can add an alias to an user on behalf of your customer. Please review the steps below on how to add alias and set one as the primary using the partner control panel.

1. Navigating to your Control Panel URL or and sign in using your administrative credentials.

2. Once logged in, Select Customer List / Editor, beneath My Customers, to show your available customers.

3. Highlight the customer the user assigned permissions needed to be added to, and click Login as Customer at the top of the Customer List.

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.