Users merged after migration

After migrating from vbulletin to Woltlab I saw that some users were merged into one. I think I figured out why. There was an option in migration settings to merge users with the same email which I left checked. I had a few users with no email address, in other words a blank email field, and all of them were merged into one.

In the posts you can still see the original user name. But the original user name no longer exists in Users. Only the one user name all of them were merged into.

My question is how do recreate those users and assign them back their threads/posts?