[Zope] Add properties to ZClass instance?

I've got a ZClass to which I'd like to add (variable) properties when the instance is created. I made a "Basic" propertysheet and tried "manage_addProperty" but Zope reports that properties cannot be added. I can add properties to DTML Documents. Can you construct a ZClass such that you can add properties to an instance, both via management screens and programatically? Thanks. -P -- Technology Concepts & Design, Inc.