Currently Being ModeratedDec 6, 2012 8:14 AM (in response to s0okz)
Unfortunately, the Ethernet ports are only active when the Extreme "extends" a wireless network.....but it is not compatible with a Belkin product for this purpose.
Ironically, the less expensive AirPort Express can be configured to "join" a wireless network and provide an Ethernet signal to the AirPort Extreme.
Frankly, this is an "iffy" type of setup that will require that you know exactly what type of wireless security setttings the Belkin router is using, so the Express will join the network correctly.
To further complicate things, the newer versions of AirPort Utility in Mountain Lion and Lion do not lend themselves well to this type of setup. You will need to use an older 5.x version of AirPort Utility for this type of configuration.
If you decide to try an AirPort Express, it might be a good idea to understand the store's return policy in advance in case things do not work out as hoped.