pixelparadies

I think this could help you:

http://xaviesteve.com/3298/wordpress-contact-form-7-hook-unofficial-developer-documentation-and-examples/#comment-137384

http://contactform7.com/2014/07/02/contact-form-7-39-beta/

For instance if you put this in functions.php you can replace the body:

function mycf7_before_send_mail($WPCF7_ContactForm) {

	$mail = $WPCF7_ContactForm->prop('mail');
	$mail['body'] = 'Hi';
	$WPCF7_ContactForm->set_properties( array( 'mail' => $mail ) );

}
add_action( "wpcf7_before_send_mail", "mycf7_before_send_mail" );

I’m sure you can achieve what you’re looking for with this knowledge.