Yes, you can do that.

However you should add it before boarding as being seen adding it during a journey could lead to a checker considering you as a fare-evader. This would particularly be true if it was the first train of a pass day’s use.

If you are in a rush for a connection and haven’t added the train then do it as soon as you are on board.

would strongly recommend adding the train ticket to the mobile pass before boarding.

Yes-the rules tell one should do it BEFore boarding.

But then there are numerous other cases-like the train you choose does not leave-staff has fallen sick-and/or is so much delayed that the ófficial´ time has lapsed mins, if not hours ago. Then you swap trains

This morning I (on quite another pass) caught a train that should have left 6 mins before-shortening my trip with nearly half an hour. This is also possible in always delayed Germany

Another use case is when you decide to leave the train earlier than originally planned. If you then want to use another train that overlaps with the first train, then you need to remove the first train and add the new one with the destination station where you left it. You can also add the overlapping train manually but that's usually more hassle.