The current approach to importing historical emails for each employee is to drag and drop them to their FYI - My Imports folder in OneDrive. These will be automatically imported to their In Tray, where they can be filed etc. Emails imported from FYI - My Imports have a Delivery Status of "Imported". Refer to Using OneDrive FYI - My Imports to Automatically Import to your In Tray.
If you need to import a lot of emails from any of your team's Outlook, please contact FYI support to discuss this.