Microsoft has announced that it will deploy the ability to add shared mailboxes to Outlook Mobile by the end of July. But if you want to see the feature early, you can join the Testflight program and install the beta version of Outlook mobile. Using Outlook for iOS with Testflight also forces the upgrade of your Office 365 tenant to the Microsoft Sync Technology.
https://office365itpros.com/2019/06/10/add-shared-mailbox-outlook-mobile/
