Hi @Gerhard Jansen !
Although you can't create a manual filter while using our global search, it does allow you to do some drilling down on the results it presents you. Check out this tutorial: Search: Finding what you need.
That being said, I will still forward your suggestion internally to our team so they can consider it. Can you tell me a little more about what type of filtering you had in mind?
Gerhard, there was another recent post which raised the same issue: