How do I add an additional sidebar to a Zen sub-theme?
You can do it with preprocess function at template.php template. I will write you just one example, and you can adapt it for your needs.

1.At Your template.php template.

function yourthemename_preprocess_page(&amp;$vars, $hook) {
  $page = $vars['page'];
// if only left sidebar is present
  if ($page['left_sidebar'] &amp;&amp; !$page['left_sidebar2'] &amp;&amp; !$page['right_sidebar']) {
    $vars['class_content'] = t('yourclassforcontentwithsidebar');
    $vars['class_left_sidebar'] = t('yourclassforleftsidebar');
 } 
// if no sidebar is present
  if (!$page['sidebar_left'] &amp;&amp; !$page['left_sidebar2'] &amp;&amp; !$page['right_sidebar']) {
    $vars['class_content'] = t('yourclassforfullwidhtcontent');
 }
}


You can find more variations here: php logical operators.

2.At your page.tpl.php, at all regions where you want flexible width, put extra class, for example for left sidebar: 

   &lt;?php if ($page['left_sidebar']): ?&gt;
      &lt;div id="left_sidebar" class="sidebar &lt;?php print ($class_left_sidebar);?&gt;" &gt;
        &lt;?php print render($page['left_sidebar']); ?&gt;
      &lt;/div&gt;
    &lt;?php endif; ?&gt;