Hi,
I'm sorry but I don't see where is the bug ?
HikaShop is not a backup solution for Joomla so there is no reason it would have a backup system of the Joomla users in it.
If you need to backup your Joomla users, there exist third party extensions for that. And if you need to backup the whole website, there also exist solutions for that.
The solution Jerome provide works, but it not as easy as clicking on a few buttons.
You need to export the tables of HikaShop via your phpmyadmin, change their prefix to the new website prefix if different by editing the file with notepad, import the table of HikaShop via your phpmyadmin on your new website database, migrate your joomla users with a third party tool, and then install the HikaShop package on the website in order to synchronize the users between Joomla and HikaShop.
So if it didn't work, it means that at least one of these steps was missing.