Hello,
Attach Serial and PDF Serial plugins can display data from various database tables.
These tables are listed at the bottom of the configuration page of the plugin instance.
When you can find the HikaShop User table, the Joomla user table is not listed.
Which means that the current version of the plugin can't display information from that table.
If you want a "fast" solution, you can use an HIkaShop custom field which would contain the Joomla username value (using an HikaShop mass-action to perform the copy and keep the synchronization).
Otherwise, it would require some development in the HIkaSerial core for a future release.
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.