Hey there,
You can add fields to it but its a little tricky. Its just a matter of duplicating the fields in the form at the bottom of the php file and then making sure that you add the parameters to link with the xml.
Im in the process of converting this to a
module:
http://www.dagondesign.com/articles/secure-php-form-mailer-script/
Which will be much more flexible and is pretty easy for you to use in the meantime as a standalone.
Ill post here when the new module is out. Probably looking at a week to two ...