Wordpress add content to rss feed

I´m trying to add some custom values to the wordpress RSS feed. For testing purposes I got this code:

function add_custom_fields_to_rss() {
        return "<test>test</test>\n";
}

add_action('rss_item', 'add_custom_fields_to_rss');
add_action('rss_item2', 'add_custom_fields_to_rss');

I have placed this to the bottom of my themes function.php. When I now try to get the rss with http://example.com/feed there is no test content which I returned in my custom function.

Does anybody know why

via Chebli Mohamed