No. You must go to a kiosk. BUT, you can modify the one you just made from your mobile app. Upon usage or expiration of this FP+, you have to go back to the kiosk for the next one. Then modify from your app if you want.
While you can modify it, there is some delay. It does not seem to show up instantly in the app. It took several minutes.
My experience with the stupid Kiosk was terrible this past Friday. We went over to DHS pretty late in the day (a little after 6:40 PM). There wasn't any line so that part was OK. For some unknown reason, the Kiosk didn't recognize my pass and my wife's as linked (even though they are and it always has them linked in the app or website).
The CM just said to make the additional FP separately. I made one for 8:55 for ToT for my wife. Then I tried to do mine and it showed no availability for ToT. Instead I made a 7:45 reservation for Star Tours. Next I went back to my wife's to make the same reservation but had to make it for 7:50 which wasn't a huge deal.
After taking a walk I decided to check the app while we were around the streets of America at about 7:05 and it said I could move my FP+ up to 7:30 (which I did). But, since my wife's wasn't linked, I couldn't see it in my app to change hers as well. I didn't notice any kiosks around so we just went over and waited in standby at 7:10. We ended up at the FP merge point at 7:27.
I told this story to further illustrate problems and inconveniences with the Kiosk method. To change my wife's reservation I would have had to trek to a Kiosk and hope that there were still 7:30 reservations available. Then, I'd spend time going through the process only to walk all the way back over to Star Tours.
Instead, it was easier to just wait on standby. Had I been able to do the additional FP+ in the app, first of all I would likely have been able to book us both for ToT since I could have made the reservation after riding Soarin' at Epcot which would have been a half hour earlier than I got to the kiosk. Then I could have ridden Star Tours on standby, grabbed a bite to eat and used the ToT FP+.
Even had ToT not been available, I could have booked Star Tours and grabbed a bite when we got to the studios. Instead, we rode ST on standby and then left the park and ate off property. As an Annual Passholder, I don't wait standby for anything that is over 40 minutes so we just skipped ToT.
Bottom line is that Disney lost food revenue due to the stupid Kiosk requirement. This won't happen with people on "once in a lifetime" trips but I'm sure it happens a lot with pseudo locals (south florida) like us that visit frequently. $10-$20 in lost food revenue every time that happens is going to add up.