Well, if you are using Dreamhost's formmail script as a backend, there's definitely no way to do it; I'm assuming that is what you're speaking of.
However, you could run the script through a first filter which may automatically add a
here or there, and then either manually have them click to go to DH's formmail backend to output "User Input+Your HTML Markup."
I do strongly suggest that you ditch DH's formmail, and take advantage of the PHP mail(); function. This would allow full customization, 200%.
You can find out more about the mail(); function here: http://us4.php.net/manual/en/ref.mail