How to use another theme's template file
Since it's always bad practice to hack core, you would have to do it with a custom module. This would be done with either template suggestion overrides or with template suggestions in a preprocess hook such as template_preprocess_node or template_preprocess_page.

Unfortunately this is not enough when you place the template file (tpl) anywhere but the theme's templates/ folder. Your custom module's template folder is not known by default so you will have to notify Drupal about. To do this, you will need to use hook_theme_registry_alter. Here below is an example from one of my projects in Drupal 7 (and it's live and works).

/**
 * Implements hook_theme_registry_alter()
 */
function CUSTOM_MODULE_theme_registry_alter(&amp;$theme_registry) {
  // The Templates path in your custom module you want to add...
  $mod_path = drupal_get_path('module', 'CUSTOM_MODULE') . '/includes/templates';

  // ... or the Templates path in your custom theme.
  // If you use your already existing custom theme's path, make sure you remember that this 
  // function exists when you alter or disable the custom theme.
  $mod_path = drupal_get_path('theme', 'CUSTOM_THEME') . '/templates';

  $theme_registry_copy = $theme_registry;
  _theme_process_registry($theme_registry_copy, 'phptemplate', 'theme_engine', 'pow', $mod_path);
  $theme_registry += array_diff_key($theme_registry_copy, $theme_registry);
  $hooks = array('node');
  foreach ($hooks as $h) {
    _CUSTOM_MODULE_insert_after_first_element($theme_registry[$h]['theme paths'], $mod_path);
  }
}


/**
 * Helper function for re-ordering arrays (needed by theme_registry_alter)
*/
function _CUSTOM_MODULE_insert_after_first_element(&amp;$a, $element) {
  if(is_array($a)) {
    $first_element = array_shift($a);
    array_unshift($a, $first_element, $element);
  }
}


I've seen people advocate (or at least suggest) the use of theme_hook() instead, but I actually find the theme registry way a lot easier to handle. Especially when you add more templates at a later stage during the development process.