Hi there,
Hikashop is already working this way so I think the issue is just the mail client you are using which doesn't interpret the "reply to" information.
Try with another client and tell us if it works.
If you really want to change this you can edit the product.php file in component/com_hikashop/controllers and replace this line:
Code:
$mail->from_name = $config->get('from_name');
with this one:
Code:
$mail->from_name = $element->email;
