If you haven't synced with GTW and you have your confirmation emails coming from GTW then you'll have to remove the members of the event (just export the program members) then delete everyone from the program, make the connection and then add them back in. What I would do in that case is run the confirmation emails out of marketo as suggested above for anyone who has already entered the program. If you don't have any additional triggers or integration (attended, no show data) moving over from GTW to Marketo you could just run the confirmation emails by a smart campaign.