What would be the correct way of adding a link to existing path into admin menu?
I think the easiest way to do this would be to create a fully fledged menu item and simply forward it on to the node/add page using drupal_goto() as the page callback:

function MYMODULE_menu() {
  $items['admin/content/add'] = array(
    'title' =&gt; 'Add Content',
    'access arguments' =&gt; array('administer nodes'),
    'page callback' =&gt; 'drupal_goto',
    'page arguments' =&gt; array('node/add'),
    'weight' =&gt; 10 // Use this property to control where in the menu the item will appear.
  );
  return $items;
}