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.

I wouldn't use Google Calendar. Instead, you can grab the ICS Yahoo sports calendars as a read-only subscription. I'm sure Proton Calendar offers it and it doesn't require connecting to Google or exposing anything funny. I don't know if Yahoo offers any public documentation on it but here's an example feed, you can tweak it by just searching for your team on Yahoo Sports and adding /ical.ics to the URL (or just guessing):

An example: http://sports.yahoo.com/nfl/teams/new-england/ical.ics