I'll try and get them to set up an option inside the plugin..... Although that could be difficult... :-)
I found the code that I think is the culprit (inside clients_convert.php) =>
$first_name = get_user_meta( $user_id, 'first_name', true );
if ( !$first_name ) {
$first_name = $user_object->get('user_login');
update_user_meta( $user_id, 'first_name', $first_name );
}
But wouldn't know how to change it without breaking it :-D