Multiple Email Domains in my o365 tenant

Matt,

     Once the second domain has been added (and verified) in your tenant, you can just add aliases to any of your users and select that secondary domain. See image below.  If you want to do this in bulk, you can write a PowerShell script to go through all your users and add whatever alias you wish to each user.  If you want to insure every new user in your tenant has aliases added as well, you'll either need to make it standard practice to add all the aliases with new users, do some sort of schedule PowerShell script job, or something similar.  Unfortunately, Office 365 doesn't let you set any sort of aliases rules (that I'm aware of) in any of the admin centers so that every user gets certain aliases automatically whenever a new users is created.