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The users migration isn't the problem, it is the passwords migration, Joomla uses a different password store type you will have to figure out and add it to the password verification on Xoops.
Allready had a successfull migration of SMF to Xoops, and im currently looking at content migration from Joomla to XOOPS because our site is migrating to XOOPS because joomla/smf are in conflicting licenses.