Hi,
The common solution will be to create an order custom field which will just be displayed in the backend.
So you will be able to access to the custom field in the order ; but not in the same place than the partner section, it will be with the other order custom fields.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.