Your sql query will be like this (i guess),
$sql = "INSERT INTO `wp_postmeta` (
`post_id` , `meta_key` , `meta_value`
)
VALUES
(
'".$POST_ID."',
'_featured_video',
(SELECT P1`meta_value` FROM wp_postmeta as P1 WHERE P1.`meta_key` = 'cb_video_embed_code_post' AND P1.`post_id` = '".$POST_ID."')
)";
In this query $POST_ID
will be your various post id...
Hope it will help you.!