In this case most of the work will be spent on parsing the messages. A few relevant things are:
- Mentions will need a record in the database, check: mention.rb and group_mention.rb.
- Attachments should also be imported as
uploadand referenced in the message markdown. Alternatively you can upload images to a temp server of your choice, add a link to them in the post and enable the setting:download_remote_images_to_local. - You still have to create categories for the channels to be able to set permissions.
- If you use SSO, you can import users from your Identity provider directly. Check: Sync DiscourseConnect user data with the sync_sso route.