Note: Please copy and paste this code in functions.php of your current theme.

    function custom_ie_registered_fields_callback( $fields ){
        /*
         * Adding new field in Export form.
         */

        $fields[] = array(
            'meta_key'  => 'wpcargo_driver',
            'label'     => esc_html__( 'WPCargo Driver', 'wpc-import-export' ),
            'fields'    => array()
        );
        return $fields;
    }
    add_filter( 'ie_registered_fields', 'custom_ie_registered_fields_callback', 10, 3 );