Hi,
j2Store must use Joomla users, like HikaShop, so you should not need to do anything in that respect.
Now, to migrate user data (like custom user fields), I suppose you might be able to do it with RO CSVI:
rolandd.com/products/ro-csvi
I would recommend contacting them with more details on what you want to do exactly and they should be able to tell you if it's possible, and if so, how.
