modifying the user registration solution pack to work this way is not a simple modification that i could give simple easy to follow steps for.
For one, the users table that is created does not have a username column, it uses the email address as the user name.
You would be better off using Security Assist to create the registration and login functionality from the ground up, this will give you the option of creating a database table that has a user name column for you to use. While running the security assist wizard, you will have the option of setting the user name column as the unique column.