Is it possible to create a RSS feed containing all blog entries? [closed]

There’s an add_feed() function. You pass it a function via the second callback parameter. So something like this should work:

add_action( 'init', 'wpse102646_all_items_feed' );
function wpse102646_all_items_feed() {
    add_feed( 'allposts', 'wpse102646_get_all_items' );
}
function wpse102646_get_all_items() {
    $args = array(
        'numberposts' => -1,
        'post_type' => 'post',
        'post_status' => 'publish',
    );
    $posts = get_posts( $args );
    $feed = '';
    global $post;
    foreach( $posts as $post ) {
        $feed .= get_the_title();
        $feed .= get_the_content();
    }
    return $feed;
}

This code is untested.

Also, I’m not entirely sure if return $feed; is what’s required by add_feed(), or if you should instead echo( $feed );. I suspect the former.

Additionally, any time you change your add_feed() code, you’ll need to flush the permalinks. The recommended way is to go to Settings » Permalinks and Save your existing permalink structure.

Reference

Codex:

• add_feed()