Is there a way to return deleted users?

[Assim]

I know there isn't an official way but is there a way to return a user who I know his user ID and I know the name he used to have before his posts are shown as guest?

Well, how can I create a new user with ID 125 and give him his old name back, and at the same time, I want to assign all his posts back to that user.

[Steve Machol]

Add a new user then run this query:

UPDATE post SET userid='xxx' WHERE username='Old Name';

...with 'xxx' being the userid number of his new account and 'Old Name' being the username on his old account.

Then rebuild the thread and forum counters in that order:

Admin CP -> Maintenance -> Update Counters