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MySQLFront is a GUI tool for managing a database that allows CSV file import, but you have to understand that XOOPS uses MD5 hash values as passwords. SO the passwords are stored encrypted (and can't be decrypted). If you can work with that, then it should be no problem.
I'd make a CSV file of a test user table, along with groups and their linking table (user linking to group), and make sure your CSV file matches the structure.
Herko