To Add another email address to a mailbox in office 365

 To Add another email address to a mailbox in office 365, you would be unable to do some from office 365/Exchange online in Exchange hybrid environment.

You have to run the following command from on premise exchange server. and then run the directorysync from directory  sync server/tool.

Set-remoteMailbox -Identity "Nawaz.Q2" -EmailAddresses@{add="nawaz.q2@pt4.onmicrosoft.com"}