Sign in with your Office 365 admin account at https://office.com/ and click on the Admin widget.
On the Active users page, choose the user.
On the user's page, next to User name / Email, choose Edit.
On the Edit email addresses page, in the text box under Alias, type the first part of the new email alias. If you added your own domain to Office 365, you can choose the domain for the new email alias by using the drop-down list. Then choose Add.
When you're done, choose Save.
Wait 24 hours for the new aliases to populate throughout Office 365.