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
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
This will open a new windows which displayed user’s Exchange properties. Go to the Email Address option from the left menu
Then click on the + sign
Define the email address type to set as well as his value. You can also set the new alias as Reply address