in the power cms admin_cms/users_login.php file, you can change the label for the email field by editing the following code:
to add a user name field would require custom coding that is not supported.
to make existing login valid for accessing power cms admin, you would need to add a UserGroupID column top your users table, set the default value for this column to 0
for any user that should be able to access power cms, set the value to 1.
in the login page, edit the authenticate user server behavior, on the third step set it to store the UserGroupID column in a session named UserGroupID