It's not really possible with usual methods as $QA_CONST_PATH_MAP only allows basic slugs, not ?sort=answers
However it's possible with a slightly hacky workaround. First set $QA_CONST_PATH_MAP as such:
$QA_CONST_PATH_MAP = array(
'questions' => '',
);
Then underneath that in qa-config.php put this:
if (!isset($_GET['qa-rewrite']) && !isset($_GET['sort'])) {
$_GET['sort'] = 'answers';
}