Please contact Entur in Norway directly to book:

https://www.interrail.eu/en/plan-your-trip/reservations/how-do-i-book-my-reservations

(Scroll down to Norway.)

Maybe the SNCF ticket office can also do it. SNCF does not do it on the phone.

From other fora/similar Qs I´ve learnt that they only as for now sell compartments for ´closed groups/families´ and will not as was usual let complete strangers share same compartment.

For seats there seem to be restrictions as to how many in 1 car.

I highly doubt if SNCF can arrange it now, but you can always try. Entur is always mentioned as about only way to do from out of NO.

We cannot find that night train in the interrail app. In the Internet, there is still that night train available. Does anybody knows what that mean or do I have to call Entur (waiting probably long in the waiting line, and then discussing a complicate issue by phone in english - would prefer mail...).
We think about taking the regional train instead, are there some experiences how comfortable and full they are (travelling start of august)?

The rail planner app should not be used to plan. It does not get enough updates to be reliable (only about once per month). For Norway, check the timetables on entur.no. For international planning, use the DB planner or DB Navigator app (updated twice/week).

In entur.no lists the train, then it exists. The rail planner app will eventually be updated, hopefully before you travel, so you can add the train from the planner. If not, then you can always add a train manually.

If you don't like to call Entur to book, then you could try DB: they should also be able to book Norwegian trains at the ticket office or by phone (030 2970).

Thanks for your answer. Mainly I want to make a seat reservation, and I cannot find on Entur.no how to do this. And is it necessary to add the train to the planner if you are able to make the reservation on entur.no?
I really doubt that DB manage to book a norwegian train, after our experience it works only for trains which start in Germany...

DB can for sure book seat reservations in Norwegian trains. But only at the ticket offices (or DB travel agents) or by phone (030 2970). The possibilities on the DB website are much more limited.

Booking a seat reservation via Entur is only possible by phone. See the link in my first comment.