Oops. The script wasn't in this forum... it was in a blog elsewhere...

http://www.mitchelsellers.com/Blogs/tabid/54/articleType/ArticleView/articleId/84/Creating-a-Standard-DotNetNuke-User-Via-SQL.aspx

Touching the core tables will change your upgrades and testing from a 1-4 hours project to a 8+ hour project.  You should not touch the CORE code or SQL Tables.  You can instead create a new table with a one-to-one relationship with the core users table that extends it with the feild that you need.

When searching for a solution to your problem of creating accounts on another DNN installation, you should consider another option that you have is moving the storage of the user data to another location (such as the same Database as your original DNN installation) through the use of custom membership providers.  You may even find a 3rd party Single Sign On solution that has done most of this work for you.

I have done this in the past, I had 4 DNN installations sharing a single DNN installation's users, but those users were assigned to very different roles from one site to another.  (The different sites were actually 3 competing companies in the same industry, using a common organization as a single sign on provider.)  The solution is no longer in use because of political and business reasons - but from a technical standpoint it worked perfectly.