I'd recommend not even trying because there are too many relationships with users and settings to other objects. If you're using version 10, go to System>Object Types and look for Users. You can see the relationship between a User and 20+ other objects. Not to mention the 60+ different FK relationships on the table as well as references in the stored procedures and views. So I'd highly recommend NOT even attempting to modify the user and settings portion. You'd be better off creating a custom module binding the user object to the custom user settings to store that data vs using the out of the box settings. This solution would have an upgrade path whereas modifying the out of the box user and user settings tables, stored procs, views, etc would not.