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Hi,
I tried to do just that yesterday, and so far seems to work fine:
1. First I changed the existing user database. I just added the necessary rows to match my needs (I guess you can make a whole new table if you wanted).
2. Then I changed the following files: register.php, include/registerform.php, and kernel/user.php (Depending on what you want to do, you need to add or delete code).
I've managed to change the writing to the database process, but haven't tried the retrieving from the database part. It should be easier...
Just remember to BACKUP ORIGINAL FILES before you try to hack them. Especially since you say you don't know much php.
Hope this helps (at least will give you an idea about where to start), but don't blame me if it doesn't (I'm new to XOOPS myself, and hadn't had change to thoroughly test what I did yesterday).