functions.php

<?php
  add_filter('aioseop_robots_meta','klausk_noindex');
  function klausk_noindex($string) {
    global $wp_query;
    if ($wp_query->is_category && $wp_query->query_vars['cat']==1) {
      $string = str_replace('noindex','index',$string);
    }
    return $string;
  }
?>