Can I add a satellite to Rbr20 and where to buy

Any of the original Orbi satellites will connect to the RBR20 router, this includes: RBS50, RBS40, RBS20. RBW30, RBS50Y, and RBS40V.

None of the "Pro" or "AX" satellites will connect via WiFi backhaul and neither will the RBS10.

Finding the original Orbi products is getting more and more difficult. eBay is one source. Amazon has some, but be very careful to ensure it is one that will connect before ordering.