You can save an Event directly from any itinerary in the Itinerary Builder to your Library very quickly and easily! After creating the new Event on the itinerary, click the ellipsis (3 dots) and then click Save to Library.
Here's what it looks like:
Just a heads up, if the Event is already a saved Library Item, you will see the option to overwrite the Library Event. This will overwrite all of the information on the Library Event with what you have on the itinerary version of the event, including ideas, notes, and photos.