How to add new items to existing menu using custom module
For Drupal 6 you do use menu_link_save:

$item = array(
  'link_title' =&gt; 'Menu title',
  'link_path' =&gt; 'menu/path',
  'menu_name' =&gt; 'menu_name',
  'module' =&gt; 'your_module',
);
menu_link_save($item);