[exim] Is there a way to add/remove headers in user filters?

Hello,
we are setting up a quite complex architecture with exim, and we wanted
to implement this workflow:

1. for each delivery, scan the message (with clamav and spamassassin)
and add (for each) a header with the result of the scan
2. for each message routing, check if teh user has defined a filter to
handle spam (and virus) messages, and in that case "do the action"
specified (that's usually add a string in front of the Subject, but can
be also mail discard or so) [the filters are read from a db, and each
user can have their own set of filters, different from the others]

Sadly, the 2. point fails because of

== <recipient> R=filters defer (-17): error in filter data: header
addition and removal is available only in system filters: near line 2 of
filter file

Is there a way for a user filter to change message headers? If yes, how?
:) if not, how would you do that?

Thanks in advance,
--
Sandro Tosi
Product Engineer
Linux based Solutions
Hosting Products
R&D | Dada.pro
sandro.tosi@???