AirPort Extreme and AirPort Express

To be able to do both at the same time you just need to reconfigure the AirPort Express Base Station (AX) to "join an existing wireless network."

Here how:

Either connect to the AX's wireless network or connect directly, using an Ethernet cable, to the Ethernet port of the AX, and then using the AirPort Utility in "Manual Setup" mode, make the following changes:

AirPort > Wireless
o Wireless Mode: Join a wireless network
o Network Name: <existing AEBSn's wireless network>
o Wireless Security: <select the encryption type of the existing wireless network>
o Wireless Password: <enter the existing wireless network password>
o Verify Password: <re-enter the existing wireless network password>

Music
o Enable AirTunes (checked)
o iTunes Speaker Name: <enter desired speaker name>
o iTunes Speaker Password: (optional)
o Verify Password: (optional)
o Click Update to write the new settings to the AX

In iTunes:
iTunes > Preferences... > Advanced > General
o Look for remote speakers connected with AirTunes (checked)

Would this work the other way around? I currently have my iMac wirelessly connected to an AX that is connected to my cable modem. I want to add a AEBS to the network (extend the network range) but not have the modem connected to the AEBS.

Tesserax wrote:
or connect directly, using an Ethernet cable, to the Ethernet port of the AX,

Tesserax. Quick question if you're out there. If I connect a new AEBS "n" to my old AX "g" as you recommend above, will I retain "n" speed on my wireless network or will the "g" AX still drag it down? Thanks.

You won't be able to have the AEBS join the AX as the AEBS does not support this feature, but you could create a Wireless Distribution System (WDS) with the two router to extend the wireless range of the AX.

My recommendation to the OP is to have the AX "join" the AEBS, not the other way around. This is because the AEBS, unlike the AX or Time Capsule, does not support this feature.

Regardless, if you connect any non-"n" device to a "n" network, the overall bandwidth performance will be impacted.