I add this at the end of qa-page-tag.php before 'return...'
$request='tag';
$request.='/'.$tag;
$qa_content['navigation']['sub']=array(
'by-newest' => array(
'label' => qa_lang('newest'),
'url' => qa_path_html($request),
),
'by-unanswer' => array(
'label' => qa_lang('unanswer'),
'url' => qa_path_html($request, array('by' => 'unanswered')),
),
'by-answers' => array(
'label' => qa_lang('answers'),
'url' => qa_path_html($request, array('by' => 'answers')),
),
'by-upvotes' => array(
'label' => qa_lang('votes'),
'url' => qa_path_html($request, array('by' => 'upvotes')),
),
);
you can make a plugin for it too
http://qa-themes.com/forums/topic/tutorial-overriding-q2a-pages