Ugh - what an unfortunate situation to find yourself in. You have my sympathy!
Maybe you can revert to a backup, if not too much new discussion has occurred since the great purge started?
Otherwise yes, I’d recommend adding the users using an import tool. I’m not sure how to do that but you could get help if you post the request in Marketplace.
Reflecting on this for my own purposes, I am glad to be reminded of the benefits of doing the reverse - using e.g. WordPress as the SSO provider, so you can benefit from the automatic purging of dormant users on the forum while retaining the users on the main site. The nice thing is that when users log in again they are added again to the forum, without even needing to do anything special. I recently was able to set up logging in by email link in wordpress, which has improved the user experience. WordPress password management is terrible out of the box.