Hi Thomas,
I'm not sure if it's possible.
Anyway, please have a look at
this discussion in the official Joomla forum.
Then you might want to search for a 3rd party extension that allows you to do that. Have a look at
this article that I found. But it seems that this extension if for Joomla 1.5 and 2.5 only.
And finally, you might want to have a look at
this article about the "Smart Search". It allows you to configure filters.