Adding text to GMail message before forward

I wanted to add a small text to emails before forward them, like a little "notice" line. I can add it to the text/html part in a HTML email, but not in the text/plain part. Here's the plan body of the message:

Content-Type: multipart/alternative; boundary=20cf303ea8aab6b10304e1b6fd7e

--20cf303ea8aab6b10304e1b6fd7e Content-Type: text/plain; charset=ISO-8859-1; format=flowed; delsp=yes

AT LAST!!!!

--20cf303ea8aab6b10304e1b6fd7e Content-Type: text/html; charset=ISO-8859-1

[This email forwarded with 'FORWARD TO MANY' GMail Gadget, more info https://sites.google.com/site/replytomany]

AT LAST!!!!

--20cf303ea8aab6b10304e1b6fd7e--

Anyone know how/if it's possible to do it?

Cheers.