You can register your own login handler. You should take a look in the App Store, for example the 'LDAP Synchronization' or 'License Pool Manager' modules do this as well.
I have looked at several modules using customised login handlers. I wish to bypass the standard login handler (not create my own) and procedure used so that I can create a form or any other method that simply takes an input of username and password and then uses those details to create the client session and registers that on the server as well. Preferably in one java action. I want to be able to log in with a client using two fields password and username the source of the username and password is not important.