The text footer is called a signature file. What you need to do is open up a text editor (like kedit) and type in what you want to appear in the footer. Save the file under a name like signature or footer. Go into Kmail and select File | Settings... Look in the "Identity" tab for the "signature file" field. Click on the ellipses button and select the file you just created. Now Kmail will automatically insert that file in any message you write.