I don't think there's a way to transfer them to the app (there's no need to anyway) but someone else may correct me.
German seat reservations are best done on bahn.com directly (way cheaper) : 4.90€ per journey (incl. multiple trains) and no booking fee. For a single train it's best to use tickets.oebb.at (3€).
Optional seat reservation prices can vary massively as you can see.
No can't be moved to DB, because Interrail sells reservations for trains in Europe with optional reservations via Belgiums Railways booking system (the price 6€ + 2€).
Next time book directly via DB App to get it there (and cost less). The price is 4,90€ per journey, instead of 6+2 per train via Interrail.eu