[EDIT: March 17 - I just want to be clear that I am not setting any Biznik policy here, or anything. Just proposing ways that the existing functionality might serve your immediate needs.]
I don't know that posting the eWN events as Biznik Events is the right solution then, if they need to go to another site to register as well.
It might make more sense to create a Discussion Topic that was "eWN Events", and each reply to the topic would be the date for the next event. This way, the latest event would always be at the top of the list. Within that post you could provide a direct link to the eWN registration form.
Alternately, each eWN event could be a separate Discussion post. "March 14th eWN RSVP List" could be the title, and each comment on the topic would be considered an RSVP.
Or, it could be that members of eWN understand that their events are not actually Free, but the Biznik RSVP tool says free anyway.
It is an interesting question, and - as you can see - we are still learning how best to use these new tools.


