How to append text to outgoing emails in qmail?
http://www.ornl.gov/lists/mailing-lists/qmail/2002/09/msg00440.html provides one way to do it - sadly by patching.
However, note the multiple caveats with this approach. Also, there are many indications that this sort of thing is hard to do in qmail (no specific tool or script) because it's "not a good idea". What problem are you trying to solve?