[Zope] creating a ZClass within a Product with Python

Hi folks, I hava a litte Problem. I have a Product written in Python, which inherits from ZClasses. Now I want to add a ZClass to the Product with Python. my first thougt was, to simply put "manage_addZclass(id)" in the Constructor, but nothing happened (no ErrorMessage, no ZClass was visible, when I clicked on the Product in the management screen) Does anybody have an idea how to do this. Or an url, where I can see an example. Thank you very much! Markus Jais