Hi Joakim,

of which train exactly are you speaking?
For Italian nighttrains (ICN - InterCityNotte) you can book a seat reservation for 3 euro if you have an Interrail Pass.
For the Nightjet operating from Italy to Germany/Austria you can book reservations for a seat for 14 euro.

For the Nightjet operated by Austrian Rail on the Routes Munich/Vienna - Rome/Milan/Venice and Vienna - Livorno you can reserve easyily at the website of austrian rail oebb.at :)

From 14€for a seat :)  as @MartinM  already said :)

 Note that due Italian Law it can happen that you have to book private compartments as you are not allowed to travel with foreign passengers This happen especially for the Nightjets from Vienna and Munich to Milan/Rome and the Nightjet Vienna - Livorno :/
Then you needed a a single slepper reservation + Interrail
or you could use the special offers of Private Seat (from 99€) or Couchette (from 199€) comparment these special offers doesn´t need a Railpass :)