You need to add an email alias to an existing Office 365 account that is sync’d to an on premise Active Directory.
Unfortunately this must be done using ADSIEDIT unless you are using Exchange On-Premise in a Hybrid mode with Office 365.
- On a local domain controller open ADSIEDIT.msc
Expand Default naming context
Navigate to the correct OU where the user is located
- Right click the user you want to edit and click Properties.
- Scroll down to the attribute: ProxyAddresses and click Edit.
- It should appear blank at first, you will need to add the current email address the user has like so SMTP:Office365test@contoso.com and the new alias as smtp:firstname.lastname@example.org. The primary email address will need to have SMTP capitalized and any other ones after that will need to be lowercase.
Exit out of ADSIEDIT and you can either wait the default 3 hours that it takes to sync new data to Office 365 or go to your DirSync server and perform a manual sync.