This could actually be quite possible by utilizing the Discord API.
Perhaps a way to do this would be to get the user to link their Discourse account to Discord using oauth. Then from there you can get all of their posts from your Discord Forums and then create the posts under their Discourse account.
Personally when I interact with the Discord API I use an API wrapper (Lot’s of alternatives btw) like disnake, as you can see the ForumChannel Object exists and from there you can perhaps then get the forums posts from there and then create these topics on discourse under ownership of the users discourse account.