My solution is like the following:
I have a system called Central, in which I manage the users and their passwords using Devise, Ruby-CAS-Server system performs the authentication against this database, and CanCan and Ruby-CAS-Client is used to manage the authorizations and authentication.
And I have several other systems, for example called Satellite, which authenticates using CAS and only have a simple User Class to hold the properties coming from CAS after authentication, and CanCan to manage authorizations, and no Devise is used.
###Central
ApplicationController in Central: