Unfortunatly there is no easy way to migrate the old GMail accounts to KMail. So first I set up the new accounts and turned on mail forwarding to the old GMail adresses. After a few days I turned off my hosted spam filter solution and made KMail my main inbox. Spam filtering is really good and I had no false positives so far. Since I receive a lot of spam on this email address, that was one major concern of mine. So I will live now without any additional filter solution.
Hopefully I can rely on this solution. GMail is still in beta, but it seems GMail has grown up now – in Australia you do not longer need an invitation to sign up.
For KMail you still need an invitation 😉