Can the 2012 AirPort Express extend non-Apple 802.11n wireless networks?

I just got a new 2012 AirPort Express. I got it primarily to extend an existing 802.11n wireless network. I have a Cisco Linksys E3000 router with a 2.4GHz wireless and a 5GHz wireless. I can join the Airport Express to either wireless network and use the ethernet LAN port on the AirPort Express to connect to the network. However, when I try to extend the network, Apple Airport Utility always says it cannot extend the network. I also tried this with a WRT54G router with default settings and again, I can join the wireless network, but not extend it. Does anyone know if the AirPort Express can extend non-airport base station networks? Did this functionality change in the 2012 model?