Hi,
You will have to edit the view "order / listing" and use code for the custom fields display, something like:
<?php
$fieldClass = hikashop_get('class.field');
$this->extraFields['order'] = $fieldClass->getField(29,'order');
if(!empty($this->extraFields['order'])){
JRequest::setVar('hikashop_check_order',1);
$this->type = 'order';
$js = '';
echo hikashop_getLayout('checkout', 'fields', $this->params, $js);
}
?>
Where you have to replace "29" by the id of your custom order field.
And then add code to save the order with the new params, so get the order data, when pressing "submit" get the value of the custom field and save the order thanks to the function in the order class.
It require good PHP knowledges.