Is it possible to lock all new and existing WordPress posts to one specific author?

This is untested, but you could use the wp_insert_post_data filter to change the post author to a specific value whenever a post is inserted or updated:

add_filter(
    'wp_insert_post_data',
    function( $data ) {
        if ( 'post' === $data['post_type'] ) {
            $data['post_author'] = 2; // Replace with desired author's user ID.
        }

        return $data;
    }
);

Just be aware that if posts are being created by users who do not have the edit_others_posts capability, such as Authors and Contributors, then they will experience unusual behaviour, because they will not have permission to do anything with the post once it has been saved. So they could see an error when the post page reloads after pressing Publish, for example.