If you don't get an answer here you might try the privacy communities. There may be a way to convert one of the other calendars to Proton.
Edit : This might not help if you have completely disabled your Google calendar or don't want to involve Google at all but I was able to add Kansas City's schedule by going here and adding the calendar to my Google account and then adding the calendar in Proton by adding a calendar and using their Google import. It's kind of lame, but I didn't have to have Google calendar installed to do it.