quattroworld

I recently picked up an airport express for two purposes: to extend my Wi-Fi network and also to stream music using airplay.
My primary router is a previous generation airport extreme.

The airport utility did all the work configuring the express to extend my Wi-Fi.
And I can also see the device in the airplay menu when it's connected to my network over Ethernet.

A friend of mine recently gave me his old airport extreme which I will use instead to extend my Wi-Fi. So now I'd like to use the airport express only to stream music over Wi-Fi without an Ethernet connection to the network.
The manual says to join the airport express network to stream to it.

But I don't want to join another network... I want the express on my existing network.

Admittedly I have not tried to do it yet. Does the airport utility configure it to join my network after I initially join the express network?

Thanks!
Peter