Hey Bardos,
Please allow me to ask few questions as I am not exactly clear about your target.
1. What are those 'Ubuntu Servers' you are mentioning?
2. True, there is nothing called 'uidNumber' attribute available for users. By default, an user has givenName, sn, userPassword, displayName, uid and few other Gluu related attributes ( i.e. memberOf, inum ).
3. It would be helpful if you can share any use case diagram with us,we would be able to understand your target.