To book reservations you can book also via Interrail.eu website (wich is also on the app), but I don’t recommend to book there, often it doesn’t work and it cost more than to book directly at the train company. 

For Italy and France you can book directly via ticket office of SBB CFF FFS. Online possible for Italy via tickets.oebb.at using ONE WAY TICKET option and adding as discount at adult “Interrail/Eurail Global Pass”. 

For Germany, Austria or Czech Republic book directly via DB, ÖBB or CD with the seat only option. 

Here a tutorial for reservations: How to get Reservations | Community

P.S. You can add your Paper Pass in the app and add the trips there, or use it the old way with writing every train on the pass with pen. Record your journeys in the app | Interrail.eu

Thank you very much Angelo. Is it possible to book only reservations at SNCF? Kind regards. 

Hey Sebastian, sure you can buy reservations for trains within France at Ticket Machines of SNCF in France and all reservations (also international like TGV Lyria) at SNCF Ticket offices or SNCF Call Center.

If you live close to an SBB/CFF ticket office they should be able to help you with reservations in several countries. 

The SBB office is unfortunately not able to make a reservation between France and Germany. So maybe I‘ll give them a call. Thank you!

DB or SNCF office can do this or also the call center of both, but there are only some seats for Pass users on TGV/ICE of the DB-SNCF cooperation (some say that DB sells all seats also for Pass users, but it is not true unfortunately). 

You can save a lot by hopping over border on local train-RES on through TGV overborder cost an awful lot  more and is also quota-controlled. Unless time is more important as money.