This is a bit out of the ordinary I post about, bit it’s an issue people deal with – myself included. It’s unfortunately not directly supported by Microsoft to add shared mailboxes in an Mobile Outlook – which is a bit odd, since users begin to access emails on mobile devices more often than before.
With a google search, I found a forum post from a Desiree Van Maasakker, who gave an walk-through on adding a shared mailbox in the mobile app.
You have to add it as IMAP account, and use the Advanced Settings:
Name: Whatever displayname you want
I can confirm this is working on my phone today. I added a shared mailbox in my Outlook app on my android device (17/02-2019).