It was really easy. I only had to add two new configfiles (1 for the topic called forum and on for the article kind) and three new xslfiles (1 for the list and the other two for the form)

Then i took my generic 'edit' script and removed a lot of stuff and now it works as 'addcomment'.

Each of the comments (=article) may have other comments as kids - and we have a little threaded forum ;-)