This has been a very long-awaited capability; adding shared mailbox on Outlook mobile without having to logon as the shared mailbox (aka using active account).
To add a shared mailbox, open your Outlook Mobile application and access the Settings to add an account, you then will be able to choose Shared Mailbox.
If you have multiple account configured, you will be asked to select which one to use to access the shared mailbox.
Next, you will enter the email address of the shared mailbox – same as when you are adding an additional user mailbox.
That’s it, the shared mailbox has been added successfully and is clearly identified as showing up as shared mailbox.
Enjoy the newfound convenience!