indyank - 3:09 am on Nov 7, 2012 (gmt 0)

After inserting the code and rebuilding with WP is done I will add some new pages, which are not existing at the moment. This pages then MUST have .html too, I don't have a choice right? It's prefixed by the code.

yes, .html extension gets appended or suffixed to all page permalink structures including the newer ones that you create.

There is a way to get around it but it gets complex.

How many HTML pages do you have currently? If there are few, you could retain and serve them as static HTML files on your new set up and go extensionless for your new pages in wordpress. I couldn't think of any other easier panacea now.