How do I add the field "Username" to Invoice?

              <fieldset class="adminform" id="htmlfieldset_billing">
              <legend style="background-color: #FFFFFF;"><?php echo JText::_('HIKASHOP_BILLING_ADDRESS'); ?></legend>

<?php echo JText::_('QJA Member').': '.@$this->element->username;?><br/>

                <?php
                  $this->setLayout('address_template');
                  $html = $this->loadTemplate();
                  foreach($this->element->fields as $field){
                    $fieldname = $field->field_namekey;
                    $html=str_replace('{'.$fieldname.'}',$this->fieldsClass->show($field,$this->element->billing_address->$fieldname),$html);
                  }
                  echo str_replace("\n","<br/>\n",trim(str_replace("\n\n","\n",preg_replace('#{(?:(?!}).)*}#i','',$html)),"\n"));
                ?>
              </fieldset>