You get an automatic one hour extra time added to a sports event and, if it is on a PTAT channel, you wouldn't miss any of the game if it went over that. It may well be that you can't set a manual timer on a PTAT channel. A better way, to make you more comfortable is to set a recording for the next program in the guide on that channel.
I did not get the extra hour of record time being a PTAT recording. I have seen that to be true with manual recordings of sports events. During playback, at 3 1/2 hours which was the scheduled time of the game, the recording ended.
I have found the best workaround for my situation is to, as you mentioned, manually record the next show after the PTAT game or if while watching the game, I press Info, choose "Showtimes", click on the "Recording Now" show, I get an "Edit Timer" option where I can extend the PTAT game recording up to 90 extra minutes.
What is weird is that I can schedule a manual recording of a PTAT show with ease as long as it hasn't started yet.