Hello,
Payment plugin are not related to the Joomla accounts.
You can register in your website and you do not need to perform anything else that fill a registration form to do that.
So I'm afraid that I don't understand your problem description, I don't see any issue but maybe a very specific and custom feature request. And in such specific case, I would recommend you to see for custom plugin development.
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.