Once you have installed and configured the rights extension in your application, you can change the table name from 'User' to any table name you want and similarly for 'username' column to any other column name.
Let say, we have table named 'Employee' with fields (id, name, email,...) and we want to use 'Employee' table name instead of 'User', 'email' column instead of 'username'.
To do so:
'modules' => array( ... 'rights' => array( 'install' => false, 'userClass' => 'Employee', 'userNameColumn' => 'email', ), ... ),
$superusers = $user->name;
$superusers = $user->email;
By doing so, the rights extension will starting using 'Employee' table instead of 'User' and 'email' fields instead of 'username'.