Hi Ted,
Thanks so much for checking out our plugins! Our import solution Event Aggregator can handle imports from a variety of external sources and file types, including CSV and ICS files and event imports from Google Calendar, Facebook, and Meetup. If you’re able to export existing data from your third-party calendar in one of the listed formats, you should have no problems bringing this data into The Events Calendar! You can read more about Event Aggregator and its features on the linked product page, or we’d be glad to answer any more specific questions that you might have about its functionality!
I hope that helps! Thanks again for stopping by, and let us know if you should have any other questions!