Unfortunately, I believe that you have to create a calendar file for each webinar. Since Mkto does not support recurring webinars, each webinar is viewed as one, isolated event. And the date section of the calendar file does not accept tokens. It would be nice if some of the information was pre-populated after you synced to WebEx, or whatever your event partner is.
Unfortunately Marketo does not officially support recurring webinars or events with the standard integration.
Despite that there are a number of customers that have found a workaround although it is not officially supported. Find the guide for the workaround here How to Setup Recurring Webinars in MarketoMarketing Rockstar Guides