Hi,
Yes it is possible to develop such thing but it requires a little more specifications.
Entering a serial in a custom field is one thing but it will be difficult for the custom field to consume the serial.
Otherwise, the first time it will work but afterwards, the "custom field check" will fail because the serial will already used (so not available for consume).
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.