Can just the Guest Network instead by extended?
Sorry, the Guest Network cannot be extended. Only the main network can be extended.
Unfortunately, if you do have the older version of the Express.....check the Model No on the side of the device.....if it's A1084 or A1088, this version of the Express does not have any settings to "extend a wireless network", so it cannot be used to extend the "main" network.
You need to have AirPort Express Model No A1264 to see the "extend" settings onboard.