Can I use a RBR50 v2 router as a satellite in an Orbi MESH network ?

Alas, the answer is "no".  Other manufacturers designed their products to be interchangeable in function.  i.e. whichever unit was connected to the internet became the 'router' and the remaining units automatically became 'satellites'.  Netgear did not.  As you have discovered, the "R" (router) device can only function as the base unit of a network, whether it is in 'router' or 'access point' mode.

For your own convenience, you can define this additional WiFi network with the same (identical) WiFi credentials as the primary WiFi network (SSID/password).  This will make the process of setting up WiFi devices easier. The problem is that WiFi devices will not roam seamlessly between the two separate WiFi networks.  Devices in the expanded coverage area are no issue (because they they do not roam).  People who carry mobile devices back and forth may notice that their WiFi performance gradually gets worse and worse and the only way to restore a strong (fast) signal is to disable WiFi and then re-enable it again.  Takes two seconds, but is annoying.