Hi,
To do so, it will require some modifications in the HikaShop core (to change how the product code is generated in the variant system).
Otherwise, I can suggest you to use the import system, it will allow you to import a file with the variants and their product code.
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.