Go into the cmssitemanager and development tab. Under system tables you can select the usersettings table(classname=cms.usersettings). This is where you can add your field. Then, in your code, set the

UserInfo object = UserInfoProvider.GetFullUserInfo(userid);

//this brings in the usersettings info as well. After that, you can get any value with the

object.GetValue("dbFieldName")

and set it with

object.SetValue("dbFieldName")

The dbFieldName will be the same name that you name the new column in the usersettings table.