Yes it is entirely possible, SNCF offers that upgrade through b-europe. You can book that with a 2nd class pass.

Marseille - Nice is always 20€ in 2nd class while the same train from Valence or Avignon could be 10€... Regular fares are quite high for this short leg as SNCF prefers long-distance customers on TGVs.

Btw Marseille - Nice can also be done on a reservation-free TER regional train, it doesn't take longer. As there is no high-speed railway they both share the same tracks. :)

Oh thanks for the info. So, if we take TER regional train from Nice to Marseille, we just catch the train without any reservation and sit where seats are available?

How about Nice to Cannes? Is it served by TER regional trains as well?
 

Yes exactly, they're usually double-decker trains. Add the train to the app before boarding and then sit wherever you want. Cannes - Nice is served by TER every 30 min (at least). You can use bahn.com for planning or check with the company, in this case SNCF.

For example a TER train on 13th July :

- Nice-Ville - Marseille-St-Charles 14:25 - 17:06

Just an observation on our trip there last year - TER in that area has roving gangs of inspectors checking for fare evaders so make sure you put your phone into airplane mode when travelling.

Weirdly this allows you to have instant access to your pass - otherwise as you go into a lot of tunnels it loses the signal, which stops you accessing the pass. The run into the underground station in Monaco was where we were challenged by the second gang on that train from Cannes to Monaco.

These gangs work by two each on the front and back entrances while the others work the carriage so nobody can leave at the next stop without showing their ticket.

This lack of access caused a period of stress as the inspectors in these gangs (6 or 8 at a time) can be intimidating - and I believe they hand out 50 euro fines on the spot. Luckily Monaco station has a signal and I was able to access my pass.in time to avoid a fine and leave the train.

Thank you @thibcabe for letting me know about TER trains that I can take without making reservations, which is a worthy saving since 4 of us are travelling.

Thank you @Yorkie for your tip on putting the phone on Aeroplane mode and the need to be prepared for ticket inspectors (so that we don’t have to stress out)

Just for info - the TER train from Nice to Marseille is normally a “traditional” style Intercity train with single level carriages, rather than the double decker local trains operating Cannes to Ventimiglia. The Marseille train should be much easier to find seats as the run through to Cannes has the half hourly local TER service picking up the local passengers.