I wish to give a new custom area having a meta key and meta value to my posts.

Presently the only method it's put into a publish is that if I enter in the publish and click on Update.
I've plenty of posts and basically want this practice area to be included to all posts instantly by having an designated meta value.

This meta value differs for every publish.

I discovered this useful: http://www.catswhocode.com/blog/wordpress-how-to-insert-data-programmatically

function add_custom_field_automatically($post_ID) {
global $wpdb;
if(!wp_is_post_revision($post_ID)) {
    add_post_meta($post_ID, 'field-name', 'custom value', true);
}
}
add_action('publish_page', 'add_custom_field_automatically');
add_action('publish_post', 'add_custom_field_automatically');

This can give a 'custom value' towards the 'field-name' within the $publish_ID.