Copy an author's cpt custom field/s to the author's normal p

function wpse_203349_copy_post_meta( $post_ID, $post, $update ) {

    if ( $post->post_type !== 'post' )

        return;



    $users_custom_posts = get_posts(

        array(

            'posts_per_page' => 1,

            'post_author' => $post->post_author,

            'post_type' => 'custom_post_type',

        )

    );



    if ( ! $users_custom_posts )

        return; // This author doesn't currently have any custom posts



    $fields = get_post_custom( $users_custom_posts[0]->ID );



    foreach ( $fields as $field => $value ) {

        if ( $field[0] !== '_' && ( empty( $value[0] ) || ! is_array( $value[0] ) ) ) // Ignore "private" fields (prefixed with an underscore or serialized data)

            add_post_meta( $post_ID, $field, empty( $value[0] ) ? '' : $value[0], true /* Unique */ ); // If the field already exists, it won't be overwritten, unlike update_post_meta()

    }

}



add_action( 'wp_insert_post', 'wpse_203349_copy_post_meta', 10, 3 );