Hi,
Xavier told you that you have to deactivate the Joomla registration in order to use the HikaShop "guest checkout".
It allows HikaShop to create internal accounts without password which are not Joomla accounts.
It is not possible to create a joomla account after the payment validation, useless you create a specific plugin and even if you want to create such plugin, I really don't see how it could be logically possible (for a global reflection, not a problem with HikaShop or Joomla.. Just no logical to implement it).
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.