It's complex, but doable. You would have to first get your letterhead created and saved as a BMP with the exact size of the window of the other letterheads. The size escapes me right now, but it shouldn't be hard to figure out. Next, you would have to put that letterhead in the template and get it into everyone's mail files. Without it in the template, people will not see it.
This was first published in May 2002