For Outlook for mobile app, it's only supported to view and edit shared Office 365 or Outlook.com calendars. At present, it's not available to add shared calendar of on-premises Exchange mailboxes from Outlook mobile app. You can check this for more details: Outlook for iOS and Android is adding your most requested calendar features.
You also can post your requirement to Outlook UserVoice, Microsoft team and more people with the same requirements will pay attention to it.