I haven't tried this with the fluent API, using the Events Manager might be a more straightforward way to do it.
first you have to have a way to retrieve your existing items tho, something like the example below. the Importer.GetExistingItems is just a dummy placeholder that should be replaced with custom logic to retrieve your existing items.
Then loop through the list of items and copy the data from your item. Your existing event data might not have all the properties (city, state, etc) so you can put in defaults with literal strings
hope this is helpful!
// you'll have to build this part yourself or use an API from the existing site if there is one
var importedItems = Importer.GetExistingItems();
//Initialize the EventsManager with the default provider
EventsManager manager =
// loop through all your content items
//Create an Sitefinity Event
Event eventItem = manager.CreateEvent();
//Set the newly created event properties from the existing event
eventItem.Title = item.eventTitle;
eventItem.Summary = item.summary;
eventItem.City = "Houston";
eventItem.Country = item.country;
eventItem.EventStart = item.StartDate
eventItem.EventEnd = item.EndDate;
eventItem.PublicationDate = DateTime.Today;
eventItem.ExpirationDate = DateTime.Today.AddDays(365);
eventItem.Content = item.content;
//Generate an URL for the content item and Save the changes.
//up to now, the item is in Draft State. We hae to call Publish method to publish it.