What are you trying to accomplish with it?
There are some ways you could, but do you need to do it for only certain email types, or all of them? What type of data are you trying to add?
In CS 2008, it is kind of tricky, but depends on your goal.
In the new Telligent Community, all email templates are MIME documents, so you just need to edit the document and add the "Header: Value" to it.