Post new threads and replys from outer scripts?

I want to post threads and replys in my forum from my little content management system for articles and book-reviews and so on. In 1.x Version I post a form with a Username and password.

Have anyone some fine ideas to post this direct from a external php-script?
Or when I post with a formular i must give a session-id and not the passwort and i dont know the session id because is a script.

Can someone help me?

Anyone a idea?