Hi,
HikaSerial does not propose such feature in his web-services but the information can be easily retrieve in the database.
Using the login you can get the joomla user id, then you can retrieve the HikaShop user id (HikaShop can have guest account so the id is different than the joomla one ; but you have the field "user_cms_id" to link them).
Thanks to the HikaShop user id, you can look in the HikaSerial serial table and list the serial assign to the user.
The best to extend the HikaSerial webservices will be to use a custom plugin ; otherwise it require some modifications in the core files so you wouldn't be able to update HikaSerial after that (or you will loose your modifications).
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.