I'd like to add another network speaker
Things are confusing since Apple calls the AirPort Express an AirPlay "speaker". So, when you say "speaker" are you meaning to say AirPort Express or are you asking about another 3rd party speaker which might be AirPlay compatible?
The Netgear extender(s) that you are asking about do not have any type of audio output like the AirPort Express has, so they would not work the same as an AirPort Express does on your network, is that is what you are asking.
. If you are looking for another device that would behave in a similar fashion to an AirPort Express, that would be an Apple TV. But....the Apple TV has only an HDMI output, so your amplifier or powered speaker would need to have an HDMI input jack.
Maybe you could provide some more details on what it is that you are trying to do, and what exact products would be involved?