[exim] changing header

Hi,

since I can't just deny messages that contain a virus (using
fetchmail), I'm trying to rewrite the Subject: header instead:

  warn    malware    = *
          add_header = Subject: *VIRUS* $h_Subject:

However, this eventually adds a second Subject: header to the mail,
instead of rewriting an existing Subject:. (Are multiple Subject:
headers allowed in a mail?)

Is it possible to rewrite headers within ACLs? How would I do that?