You can specify a custom widget like this in plone.autoform 1.4 and above.
In earlier versions of plone.autoform you need to specify a "field widget" (a callable that produces a widget when called) instead of a widget class. (plone.autoform 1.4 supports either one, and some other things covered in its documentation.) In this case you would use z3c.form.browser.radio.RadioFieldWidget.