To sit in 1st class, you need a 1st class pass.

If you mean that overnight NIghtJet then it will also show other options like couchette or sleeper bed. Because this system does not sniff what type of ticket you already have, it will show all options available on that day for that train. To the immense utter surprise of many people only used to airplanes, it is indeed the case that the ticket (your proof of having paid for the ride) and the extra like REServ or whatever is separate-and can hence also be changed later quite easily without the cumbersome way of cancel, payback etc.

This is certainly possible for TGV travel, see image. Then you pay an upgrade. Here this is an extra of 12 euros, for this trip.

I don't know about other train companies. You will have to ask their customer service.

@Sven wich website/app is this to book the upgrade or reservations for SNCF?

This looks like Belgian railways. Interesting option.

This is the website of B Europe, Nmbs

Curious if it also works for other companies? Such as THALYS, DB BAHN, TRENITALIA,.....

I suppose that depends on the operator.

For Thalys, you can also check via b-europe.

For DB, you'll have to pay the difference between the 1st and 2nd class fares (with BahnCard discount, if you have one). No idea for Trenitalia.

It was possible via Italia Rail, but not via Trenitalia. I don’t know now. 

But the Interrail rules of Trenitalia says not valid in Premium or Executive class. This was also possible to upgrade.

I looked for THALYS on the B europe site, and don't see any options to buy an upgrade for 1st class if you have a 2nd class interrail. Too bad this is so different from company to company.