ZEN - Adding New System Defined Widgets

Here's some relevant documentation, if I'm understanding your question correctly: http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=GZAP_customization

There are two topics covered here: custom Zen components, which have their own HTML rendering, and composite components, which are simply reusable groupings of other Zen components.

These really wouldn't be "system-defined" - you'd have them in a separate XML namespace and they'd be distinct from the "system-defined" Zen component classes, but they can be used in the same page alongside standard Zen components.