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I had the same problem. For my situation, I need users to provide a first name, middle initial and last name, instead of a single display name.
Since I'm using 2.2.3, I was able to add custom profile fields easily. But I still had to do some hacking to the profile module to remove the display name, and the "real name", from the forms, and auto-populate the display name field from the added custom fields.
And then I had to add some other fields, such as SSL client certificate info, that require special processing during registration and login.
Hopefully this will be more flexible in XOOPS 2.3.