[Exim] router that only adds a header ?

Hi !

Im new to this list but haven't found the answer to my question anywhere
in the archive:

Is there any way to add a router that does nothing but adds another
header to the mail message (no transport, no delivery)?

My intention is to include a MySQL-based blacklist, so that all mails
passing this router will be tagged - they are sorted by another router
(that does other things, too) later on - this works already fine.

There may be other ways to create such a behaviour, but I think for my
intentions this way would be best.

Sorry if this question was asked before and I just wasn't able to find
it ...

Martin