Yeah, what you need to do is register a user - then change that user's number.
UPDATE w3t_Users
SET U_Number = xxxx
WHERE U_Username = 'yyyyyyyy'
Or - you can set those posts to anonymous - so if you know user #506 is missing, you can do a query like this:
UPDATE w3t_Posts
SET B_PosterId = 1,
B_Reged = 'n'
WHERE B_PosterId = 506
That'll set all posts by #506 to an anonymous post.
Hope that helps.