Magento won't find the controller as it would search for Mage/Customer/AccountController.php. I guess the only way is in fact require_once a controller php class file. I think there's a complete solution: http://inchoo.net/dev-talk/how-to-extend-magento-core-controller/

The other, much less elegant way is to copy the whole controller you want to extend, and let it extend Mage_Core_Controller_Front_Action.

Edit: Or Mage_Adminhtml_Controller_Action for admin area controller.