The supplement for the Paris-Berlin sleeper (or any operated by City Night Line) with a railpass is fixed at €65 if sharing a compartment, or €105 for a single-use compartment, and can be booked easily on-line on the DB (German railways) website.
For getting around within cities, remember a railpass will generally only cover the train services provided by the national operator (for example, S-Bahn trains in German cities), and not other forms of transport (trams, metro or U-bahn). Consequently, it is of limited use. You will find this especially in Rome - you would be unlikely to use the local train service (except if you did fly instead - to go to/from the airport).