I believe you are asking if you can "daisy-chain" an additional Express by connecting it to an existing Express that is also connected to the Extreme - is that right?
I don't think that will work. Apple's KB articles indicate that all hardwired base stations must be connected to the primary base station (your Extreme) either directly or though a switch. See the pictures here:
The circled black box in the above is an Ethernet switch, which you need when you run out of LAN ports on the Extreme. They are not expensive, $25 or less for a good one.
Make sure you configure all the Expresses as bridges. See the highlighted fields in the following: