Hi,
For the users, it's not really a HikaShop problem but a Joomla problem. You'll find several extensions allowing you to import/export users between Joomla 2.5 and 3.x
www.google.com/search?q=Joomla+migrate+u...ceid=chrome&ie=UTF-8
Once you have all the users in Joomla, export/import the hikashop_user hikashop_order hikashop_history hikashop_order_product tables via PHPMyAdmin.
Finally, click on the "check database" button of the Hikashop configuration to synchronize the Joomla users with HikaShop's and fix the tables if necessary and you should be good to go.