问题将是设置帖子缩略图（附件ID）所需的数据与自定义字段中可能存在的内容（URL）之间的差异。

我建议的方式非常草率：在WordPress中添加更多附件。更好的方法可能是编写自定义SQL查询，从自定义字段URL获取附件ID。这很有效，但可能不是最好的方法。

首先，获取所有帖子：

// get an array of all the posts
    $posts = get_posts(
        array( 
            \'numberposts\'   => -1,
            \'post_type\'     => \'post\'
        )
    );

    // bail if our get_posts() call failed
    if( empty( $posts ) ) return;

然后我们将遍历它们，获取原始图像URL，然后将其作为新附件插入。最后，我们将更新Posteta表以反映新的缩略图ID，并去掉旧的post缩略图自定义字段。

foreach( $posts as $p )
    {
        // change your old thumbnail key!
        $image_url = get_post_meta( $p->ID, \'your_old_thumbnail_key\', true );

        // no thumbnail? on to the next
        if( empty( $image_url ) ) continue;

        // find our mime type for later
        $filetype = wp_check_filetype( $image_url );

        // Set up an array of args for our new attachment
        $args = array(
            \'post_mime_type\' => $filetype[\'type\'],
            \'post_title\'     => esc_attr( $p->post_title ), // you may want something different here
            \'post_content\'   => \'\',
            \'post_status\'    => \'inherit\'
        );

        // Insert the attachment!
        $thumb_id = wp_insert_attachment( $args, $image_url,  $p->ID );

        // gotta set up some meta data (height, width, etc)
        // our functions are int his file, so we have to include it
        require_once(ABSPATH . \'wp-admin/includes/image.php\');
        $metadata = wp_generate_attachment_metadata( $thumb_id, $image_url );
        wp_update_attachment_metadata( $thumb_id, $metadata );

        // Finally! set our post thumbnail
        update_post_meta( $p->ID, \'_thumbnail_id\', $thumb_id );

        // Uncomment the next line to delete the old custom field
        //delete_post_meta( $p->ID, \'your_old_thumbnail_key\', $image_url );
    }

就是这样！

这里是一个插件：http://pastie.org/2386814