With the new Office 365 there is some actions which have changed to manage user properties.

As part of this, adding an email alias is now more simpler.

Logon to the Office 365 administration portal with an administrator account

Then go to the Users and Groups section from the left menu

image

On the Active Users tab, select the user you want to add email aliases and click on the Edit Exchange Properties link on the right side

image

This will open a new windows which displayed user’s Exchange properties. Go to the Email Address option from the left menu

image

Then click on the + sign

image

Define the email address type to set as well as his value. You can also set the new alias as Reply address

image