Anything is possible, it is just a matter if how much time, money and effort you want to put into developing and maintaining a system. I have seen a case where the solution was to use a custom written password change tool that then search all directories (Windows, LDAP, NIS, etc.) in an organization, search for an existing user account, and update passwords. So each OS was able to work in a native mode, but you still only had a single username/password.
I think the new direction I have been seeing is for a single backend authentication mechanism that all systems will query directly.