[exim] Adding Headers in a filter

Hi, list!

I'm trying to write a filter for Exim 4.67, that, if some conditions are true,
it adds an Header to the E-Mail.
Unfortunately, Exim says, I can do that only in System-Filter.

Has someone an idea, how can I force Exim to do what I want?

Thanks
Luca Bertoncello
(lucabert@???)