Hi,
Without more information about your Joomla modification it would be difficult to answer you.
HikaShop could by synchronize with the joomla users, it would be use the "joomla user id" as identifier and not the email.
The "user_email" in HikaShop does not have restriction in the database, so you would be able to set several time the same email for different users.
But it is possible that modifications in HikaShop core would be required. It's not something which is generally possible so, some security check could be present in HikaShop.
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.