function wpf_set_additional_roles( $user_id, $action ) {

    $user = new WP_User( $user_id );

    if( isset( $_GET['add_role'] ) ) {
        $user->add_role( $_GET['add_role'] );
    }

    if( isset( $_GET['remove_role'] ) ) {
        $user->remove_role( $_GET['remove_role'] );
    }

}

add_action( 'wpf_api_success', 'wpf_set_additional_roles', 10, 2 );