Добавление новых полей в форму на странице заказа товара

/* Сохраняем данные, введенные на первом этапе */
....
$new = to_int($_POST['new']);

   /* Пишем информацию об адресе доставки в сессию */
...
  $_SESSION['new'] = to_str($_POST['new']);

<form name="address" id="address" method="POST">
         <h1>Адрес доставки</h1>
         <p>
            <a href="{shop/path}cart/">Корзина</a>
         </p>
         <table>
            <tr>
               <td>Страна:</td>
               <td>
                  <select id="country" name="country" onchange="doSetLocation(this.options[this.selectedIndex].value, '{/locations/shop/path}cart/')">
                     <xsl:apply-templates select="country"/>
                  </select>
               </td>
            </tr>
            
            <tr>
               <td>Индекс:</td>
               <td>
                  <input type="text" size="5" name="index" value=""/>
               </td>
            </tr>
            
            <tr>
               <td>Область:</td>
               <td>
                  <xsl:variable name="country_id" select="/locations//country[@select = 1]/@id" />
                  
                  <select name="location" id="location" onchange="doSetCity(this.options[this.selectedIndex].value, '{/locations/shop/path}cart/')">
                     <option value="0">..</option>
                     <xsl:apply-templates select="location[@parent = $country_id]"/>
                  </select>
               </td>
            </tr>
            <tr>
               <td>Город:</td>
               <td>
                  <select name="sel_city" id="sel_city" onchange="doSetCityArea(this.options[this.selectedIndex].value, '{/locations/shop/path}cart/')">
                     <option value="0">..</option>
                     <xsl:apply-templates select="city[@parent = location[@parent = $country_id]]"/>
                  </select>
               </td>
            </tr>
            <tr>
               <td>Район города:</td>
               <td>
                  <select name="sel_city_area" id="sel_city_area">
                     <option value="0">..</option>
                  </select>
               </td>
            </tr>
            <tr>
               <td>Улица, дом, квартира:<br/>
               (город, район, если не выбраны)</td>
               <td>
                  <input type="text" size="30" name="full_address" value=""/>
               </td>
            </tr>
            <tr>
               <td>Cтанция метро:</td>
               <td>
                  <input type="text" size="30" name="metro" value=""/>
               </td>
            </tr>
            <tr>
               <td>Дополнительная информация:</td>
               <td>
                  <input type="text" size="30" name="other_info" value=""/>
               </td>
            </tr>
            <tr>
               <td>Желаемое время доставки:</td>
               <td>
                  <input type="text" size="30" name="my_time" value=""/>
               </td>
            </tr>
            <tr>
               <td>
                  <input name="step_2" value="Далее &gt;&gt;" type="submit"></input>
               </td>
            </tr>
         </table>
      </form>
      

/* Сохраняем данные, введенные на первом этапе */
      $country = to_int($_POST['country']);
      $location = to_int($_POST['location']);
      $city = to_int($_POST['sel_city']);
      $city_area = to_int($_POST['sel_city_area']);
      $metro = to_int($_POST['metro']);
      $other_info = to_int($_POST['other_info']);
      $my_time = to_int($_POST['my_time']);

      /* Пишем информацию об адресе доставки в сессию */
      $_SESSION['country'] = $country;
      $_SESSION['location'] = $location;
      $_SESSION['city'] = $city;
      $_SESSION['city_area'] = $city_area;
      $_SESSION['index'] = to_str($_POST['index']);
      $_SESSION['full_address'] = to_str($_POST['full_address']);
   $_SESSION['metro'] = to_str($_POST['metro']);
      $_SESSION['other_info'] = to_str($_POST['other_info']);
      $_SESSION['my_time'] = to_str($_POST['my_time']);