5
If your needs are not too extensive, you might be able to repurpose some exiting fields that you aren't using. For example, you could change the "ICQ" field to a memberid or member number. This only requires changing the language files (language/english directory - search for ICQ).
As already mentioned, better solutions are coming, so if you can use this for now, I'd do that.
Be aware that any changes you make to the XOOPS core, whether modifying just a language file, or hacking the user object, will be overwritten if you upgrade XOOPS in the future. That means you have to keep redoing your changes with each update.
Dennis