[Zope] newbie question CMF/Plone

Hi, Can anybody tell me how to edit a portal_type (like File) to be able to add new properties (metadata) (for example: "geofocus") to the file using CMF/Plone ? Simply adding additional metadata doesn't seem to work. I tried modifying the File.py and the DublinCore.py scripts to include additional entries, and the file_edit_form as well. But when a try to add a File type, it returns the message : Dating not found in 'request/geofocus | here/Geofocus' What else should I do ? Thanks, Fabien.