[Exim] Director that only issues "headers_add"

I'm trying to write a director under Exim 3.35 (Debian) that does nothing
but adds a header. I've tried several things to accomplish this but they
all produced undesirable side-effects (new_address=, driver=pipe, etc.). Is
there something like a "driver = noop" available?

Any advice appreciated. Thanks. - John R. S.