Unfortunately the Scheduler doesn't support that at the moment.
There are two options:
1. You can implement the queue using a special instance of the Scheduler (i.e. one resource, set the row header width to 0, one time cell, use custom sorting for ordering the items...). That way you will be able to drop it back in the queue.
2. You can use a context menu ("Unschedule") and use JavaScript to remove the event from the Scheduler and render it in the queue.