Hi,
Well, if you do a translation override of the text there, you can add the extra text and link to the translation.
However, it will always appear.
Regardless of whether you're logged in or not.
If you want to display it only if you're not logged in, then you would have to directly edit the PHP file plugins/hikashop/cartnotify/cartnotify.php
There, you would change the line:
'text' => JText::_('PRODUCT_SUCCESSFULLY_ADDED_TO_CART'),
to something like that:
'text' => JText::_('PRODUCT_SUCCESSFULLY_ADDED_TO_CART'). (JFactory::getUser()->guest? '<a src="link">text</a>'),