Hi,
Because HikaSerial have an automatic processing ; if you want to attach a serial to an order manually, you could lost this serial if you click on the "refresh association" button for example.
So, yes it could be possible to assign manually a serial number to an order but it will require some improvements in HikaSerial in order to keep the information that the serial as been added manually ; in order to not "touch it".
That's why, for the moment, If you want to add something manually to an order ; i will more recommend you to use an order custom field (thanks to HikaShop business).
Regards,
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