This hook transfers all meta fields. Convenient to use for adding custom meta fields:
add_filter('avalon23_get_all_meta', function($metas, $filter_id){
if ('1' == $filter_id ){
$metas[] = array(
'title' => 'Your Custom Meta',
'meta_key' => 'test_test',
'meta_type' => 'text',//number, calendar, text
'notes' => array('test1' => 'Test 1', 'test2' => 'Test 2')
);
}
return $metas;
}, 10, 2);
Result: 
