I also had no trouble adding 3 Simpler Networks-branded "HP200PT64BT" devices to 2 TalkTalk-branded Huawei "PT200AV" devices, by plugging in the new units one at a time, each time pressing the button on one of the PT200AV units and then on the new unit. It didn't matter that the previous owner of the new units had already paired them into a network: they were still able to join our existing network.
(In our case we needed to do this because a third PT200AV had failed, apparently due to a power surge. Resetting it didn't help, and its transformer became audibly louder; I suspect it was no longer able to maintain a stable enough voltage for the embedded processor to function very long, so we needed to replace it. This was after we'd had it plugged in next to the router for 1 year on 24/7 plus unknown use from previous owner, but still we wanted to try a different model to see if it would be more robust. We only needed three in total, and someone was selling a set of three so we bought these thinking it wouldn't matter if we were unable to link them to the two PT200AVs, but in the interest of science I tried linking it anyway and it worked. I should also point out though that the reason why we wanted powerline networking was merely because we're in a flat where Wi-Fi interference seems higher than powerline interference: the outgoing connection is still copper DSL, so we haven't needed the full 200Mbps and therefore can't really comment on speed, except to say short bursts of local-network traffic worked when we tested it.)