This should work. My iMac G5 is connected wirelessly to a nearby wifi access point (actually a Siemens Speedstream Powerline access point, which connects through my power line to another Siemens Powerline ethernet plug, which connects to my Linksys router) for internet and network connectivity, but it simultaneously connects to an Airport Express in another room for playing iTunes through my audio system (an integrated tube amp, if you are curious).
It isn't always smooth sailing, though. The network signal to the Airport Express periodically, and very annoyingly, fluctuates dramatically, causing Airtunes to skip every minute or so.
I can't blame the complicated network setup, however. In my effort to debug this issue, I've actually tried disconnecting everything but the iMac and the Airport Express and moving the iMac to within 5 feet of the Airport Express. Sadly, to no avail...
But, other than this very strange behavior, which I am still trying to debug, this should work fine.
One other learning: a nearby microwave will drastically reduce your Airport Express signal strength when it's cooking.
Hope that helps.