To clarify, your current AirPort Express 802.11n is not compatible with the Belkin router for the purpose of extending the Belkin wireless network using wireless only.
If you have your existing AirPort Express connected to the Belkin router using an ethernet cable....and...you have configured the AirPort Express to "create a wireless network", you can "extend" this network if you add another AirPort Expess"n" and configure the new device to "extend a wireless network"
You do not need an AirPort Extreme as the base station in order for an Express to "extend" the wireless signal. Any Apple router can 'extend" the wireless created by any other Apple router.... a Time Capsule, AirPort Extreme , or AirPort Express as long as all devices are "n" devices.