Programatically add options to “add new” custom field dropdown

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You cannot do that with pure PHP, because the fields are fetched from existing fields, and there is no hook. But you can use JavaScript, check if the post type supports custom fields and the field does not exist already – and insert it:

<?php # -*- coding: utf-8 -*-
/* Plugin Name: Extend custom fields */

add_action( 'admin_footer-post-new.php', 'wpse_98269_script' );
add_action( 'admin_footer-post.php', 'wpse_98269_script' );

function wpse_98269_script()
{
    if ( ! isset ( $GLOBALS['post'] ) )
        return;

    $post_type = get_post_type( $GLOBALS['post'] );

    if ( ! post_type_supports( $post_type, 'custom-fields' ) )
        return;
    ?>
<script>
    if ( jQuery( "[value="demo_data"]" ).length < 1 ) // avoid duplication
        jQuery( "#metakeyselect").append( "<option value="demo_data">demo_data</option>" );
</script>
    <?php
}

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