Add a sharedmailbox in as a "full mailbox" (New Outlook)

If you have access to a shared mailbox but need it to behave as a full mailbox (e.g have access to it's settings, signatures etc...) you can do so in the New Outlook Experience . 

 Click the settings cog   

 

 Then under Accounts > Shared with me 

 

 You can show/hide mailboxes here as well as choose  Convert  next to any mailbox to convert it to a full mailbox, with access to it's settings and signatures. 

   

 After a restart of Outlook you can see it now behaves as if added in via username/password.