You can add other satellites. It doesn't have to be the wall plug variant.
The only thing to keep in mind is that the RBR20 router that comes with the RBK20W bundle uses a 2x2 866Mbps WiFi backhaul. Some satellites, like the RBS50, are capable of linking with a 4x4 1733Mbps WiFi backhaul. These sats will link with the RBR20 at 866Mbps max.
