Link To Already Defined Path In Menu Using hook_menu()
You can best use hook_menu_alter(), which should be used when "Additional items may be added, or existing items altered."

In custom_node_add.module :

function custom_node_add_menu_alter(&amp;$items) {
  if (aray_key_exists) { //some sanity checking;
    $items['node/add']['page callback'] = 'custom_node_add_page'
  }
}

function custom_node_add_page() {
  return t("Hello World! This is content on my node/add page");
}