Right. If the user doesn’t exist in WordPress, they can’t log in to WordPress.
You don’t need to impersonate anything.
If you want WordPress to be your authentication source
- create all of the users in WordPress. See that the email address in WP matches Discourse
- switch Discourse to using WP as DiscourseConnect host
You’ll have to get people to reset their passwords in WordPress.
What problem is moving authentication to WordPress going to solve? I think you’re going to be sorry that you did this.