Adding a link is the easy part. The complex part is adding some logic into it :)
Anyway, to add a link just create a layer and add this method to it:
public function initialize(){
if ($this->template === 'questions') {
$newSort = 'new-sort';
$newTabSelected = qa_get('sort') === $newSort;
$this->content['navigation']['sub']['recent']['selected'] = $this->content['navigation']['sub']['recent']['selected'] && !$newTabSelected;
$this->content['navigation']['sub']['my-id'] = array(
'label' => 'Link text',
'url' => qa_path_html('questions', array('sort' => $newSort)),
'selected' => $newTabSelected,
);
}
parent::initialize();
}
Piece of cake!