Bruno,
You could either include the form in the template (directly or using
INCLURE)
I use this for this page, for example:
http://www.taize.fr/en_article78.html
or use this "contrib" to include a PHP script directly in the text of an
article:
http://www.spip-contrib.net/article451.html
I use this technique on this page, for example:
http://www.taize.fr/en_article82.html
I don't know if there are any other ways...
Paolo