I need a extention that add a 2nd category description at the bottom af all categorypages. Does any know a extention that does that?
You actually don't need an extension for this - you just need to move the elements around the page. Here's a post that might be helpful: http://magento.stackexchange.com/questions/40335/how-to-get-category-description-after-product-listi...
You can try this extension - https://amasty.com/meta-tags-templates.html
It can set up additional description that will be seen on each subcategory. Templates can be created on a category level, so that you can adjust the 2nd description to your needs.
About a year ago I faced the same problem. I solved it with the help of SEO Meta Templates (http://www.mageworx.com/magento-seo-meta-templates-extension.html) extension and I can suggest it to you.
It absolutely fits your needs. You can create a template for a certain category to set the second description of your own.