Simple, open your mail in the designer and go to the Subforms list. From there, you can change the header subforms as much as you want (still have to find out which ones they are, though !).
Unfortunately, all your efforts will be useless if you are hoping to send mail with your modified headers. Any modification you do will only be available to you in your mail. Whatever mail you send will arrive in standard format in the recipients mail box. For that reason, you want to avoid adding unregistered headers, since that is likely to display error messages in the recipients' mail box.
In my opinion, you're setting yourself up for a lot of trouble for very little gain.