add AJAX to an existing node/page path?
There are many ways to add scripts to an existing page, one option is:

/**
 * Implements hook_init().
 */
mymodule_init() {
  if ($some_condition) {
    drupal_add_js(drupal_get_path('module', 'mymodule') . '/mymodule.js');
  }
}


hook_menu() probably isn't going to help you here, because each menu router item can only have one page callback.