我设法让它工作，下面是答案：
-在将更新新货导入自定义字段时：\\u custom\\u stock\\u placeholder
-仅更新此字段：\\u custom\\u stock\\u placeholder
-将此代码添加到函数中。php

add_action(\'pmxi_saved_post\', \'post_saved\', 10, 1);

function post_saved($id) {

$original_stock = get_post_meta($id, \'_stock\', true); 
$new_stock = get_post_meta($id, \'_custom_stock_placeholder\', true);

$combined_stock= $original_stock + $new_stock;

update_post_meta($id, \'_stock\', $combined_stock);

}