If you had one single login portal, you could view these individual programs and see how they "login" users. They most likely create cookies with certain pieces of information used to authenticate the users.
Your login portal would have to first have some way of authenticating the users (e.g. a user database you create). Then, upon a successful login, your portal would create all of the individual cookies that subsequent programs look for. It will be as if they had visited the login interface for each program and received the authentication cookie(s).
Alternatively, you could simply create your own authentication system and modify each program to look for your own mechanism instead, though this would probably be more coding- and thought-intensive than the above method would be.
