[Mimedefang] Accessing Subject in SpamAssassin line?

[Russ GilmanHunt]

I'm converting from a fairly "pure" SA installation to a MD->SA
installation, and I've about 25 users to support (nothing too
strenuous). Many of them have a filter set up for "SPAM" in the subject
line. How do I add "***SPAM***" to the subject line of the emails? I've
tried  this:

action_change_header("Subject", "***** SPAM ***** $Subject");

But it doesn't seem to be pulling in any Subject line.  Is there a
global variable set at that point?

Thanks. 
Russ