First of all, do not modify any core files! You need to create a module, and then make your changes in your module.
I believe that the <head> section is a list. So to add your new template to the list you would put:
* Head list children
* @author Qualiteam software Ltd <firstname.lastname@example.org>
* @copyright Copyright (c) 2011-2014 Qualiteam software Ltd <email@example.com>. All rights reserved
* @license http://www.x-cart.com/license-agreement.html X-Cart 5 License Agreement
* @link http://www.x-cart.com/
* @ListChild (list="head", weight="100")
at the top of your template file.
Then you can include your link in the template like this:
<link href='//fonts.googleapis.com/css?family=Open+Sans' rel='stylesheet' type='text/css'>