I would like to make an authentication system where users or an administrator can choose which login system they prefer. The problem is that different systems have different Login-Systems and different client-informations. 
I thought I will make an simple User-Class in the c++ application and an administrator can extend this class with its own one or more User-Login-Systems in Python. Off course this service runs on a server.
How can I organize the different Login-Systems on the server and automatically use the prefered Login-System with the correct user-information class on the client application?
Assuming that you would like to support login systems such as User/Pass, LDAP, OpenID, Oauth etc.. you have to model your authentication layer to be able to support all these mechanisms. I usually consider the above authentication methods as strategies.
Lets say you have an Authentication class with an authenticate method which accepts an object that implements an interface "AuthStrategy"  and the various authentication methods can implement this interface.
Hope the object model is clear.
handling multiple login systems