Check cmn_users_write.xml might be better than the read file as it show examples.
<User userLanguage="English" userStatus="Active" userName="xoguser9" userType="Internal" externalId="2323" userLocale="fr_FR" userTimezone="America/Los_Angeles"><PersonalInformation lastName="xoglastname9" firstName="xogfirstname9" emailAddress="xog9@niku.com"/><General addedBy="admin" addedDate="2001-09-05"/><OBSAssocs>
Very easy and simple to do with a direct update to db, but that is not supported.
Better to do it in a supported manner, because that is not complicated.
Just have the minimum required fields and the timezone when you are updating/changing.