In this article, we will see how you can setup additional mailboxes in the latest version Outlook 2016. It’s pretty easy. Add Another Mailbox in Outlook 2016.
Edit: Nevermind - this KB says Send As Distribution and Security groups won't work in Outlook 2011/2016 on mac: ------- In Outlook 2016 Mac, you can go into Preferences > Accounts under personal settings > Advanced Button (bottom right) for Account you wish to edit > Delegates Tab at top > Add account under People I am a delegate for > search for distribution list/group/person. The from field should show up after setup on replies and new messages. I was able to test successfully with a shared mailbox, but not a distribution list; not sure if it's because something isn't working on my end or not. We're O365 users as well, but I tested on my personal Mac. Hope this helps! Edit: Nevermind - this KB says Send As Distribution and Security groups won't work in Outlook 2011/2016 on mac: ------- In Outlook 2016 Mac, you can go into Preferences > Accounts under personal settings > Advanced Button (bottom right) for Account you wish to edit > Delegates Tab at top > Add account under People I am a delegate for > search for distribution list/group/person.
The from field should show up after setup on replies and new messages. I was able to test successfully with a shared mailbox, but not a distribution list; not sure if it's because something isn't working on my end or not. We're O365 users as well, but I tested on my personal Mac. Hope this helps!
Type in the name of the shared mailbox, then select the desired user from the result list and click Add Select OK to close the accounts window After a short period of time that shared mailbox will appear as a folder in the View list on the left side. Outlook for the PC will automatically mount a Shared Mailbox after the next synchronization of the Outlook Address List, typically within 24 hours. 
Adding a Shared Mailbox in Outlook 2016 for Mac.