Adding on for the current day, somewhat unbelievably, is NOT possible from the app, and must be done at an in park kiosk.
I have found the app to be glitchy and unreliable at best! Restarting your phone / tablet seems to clear up MOST of the issues in the app, but be prepared to need to do this several times a day if you make any kind of actual use of the app, say to repeatedly check on the times for your FP+ bookings, try to bump up their times, and keep track of your ADR's. In other words, if you use the app the way they intended for it to be used, you will be restarting your device multiple times a day to keep said up functioning.