Extending Plone Content Types

With archgenxml and plone you can start adding new types of content easily. If you want to subclass an existing class just follow the steps here:
http://plone.org/products/archgenxml/documentation/how-to/subclassing-atcontenttypes/?searchterm=atcontenttypes

And to refer what class you can use just browse at:
http://api.plone.org/Plone/3.0/public/frames/products/ATContentTypes/index.html

Be aware that the class name of the stub have to be the same as defined in the api. For example for the news item, it is ATNewsItem (IT IS CASE SENSITIVE). And walah... it works.. :)