There are several ways to do this, but the safest one is to create a new advanced theme following the instructions here:
In your theme's qa-theme.php file, override the nav_list() method to insert the link:
class qa_html_theme extends qa_html_theme_base
function nav_list($navigation, $navtype)
$navigation['custom']=array( 'url' => 'http://your-page/', 'label' => 'your-label');
It's planned to make this much easier to do in future, via the admin interface.