Can a group of Eurail pass holders and direct ticket holders get reservations in the same couchette?

Hello, we are a group of 4 travelling from Paris to Tabres on the SNCF night train. We all have Global Eurail passes. Two friends would like to join us on the trip by buying direct tickets from SNCF.

Is there any way we can book seat reservations so all of us can get places in the same 6-bed couchette? As far as I can see the Eurail website allows to add only passholders. 

Thanks!