How can i add a custom shortcode to core shortcodes?

[rica-carv]

How can i add a custom shortcode to core or other plugins shortcodes, at theme level rendering?

For instance, i want to add a counter to the news, but i can't make it work inside the theme.
How can i do this?

class spt_shortcodes extends e_shortcodes {
  function __construct(){
    $query = "SELECT n.news_category, COUNT(*) FROM #news AS n GROUP BY n.news_category;";
    e107::getDb()->gen($query);
    while($row = e107::getDb()->fetch()){
      $data[$row['news_category']] = $row['COUNT(*)'];
    }
    $this->cat_count = $data;
	}

function sc_news_cat_count(){

  $parm = $this->news_item['category_id']);
  return $this->cat_count[$parm];
}
}

$parm is always 0, because i can't get the category_id from current news template at rendering time, but the shortcode output is rendered in the news_template that is inside the theme.... weird...