Can AirPort Time Capsule ethernet ports be used if it's the extended base of a primary WiFi network?

It will only work as Extend wireless to another apple wireless router. If that is the case then yes, the ethernet ports will work.

Bottom line.....Do not buy the Time Capsule unless you already have another Apple router that is set up to provide the existing wireless network.

If you do have an Apple router providing the existing network.....(we are still not out of the woods yet).....what is the model number of the device that you have?

If you are not sure, look on the bottom of the device. The model number is usually embedded in the foam pad....will be something like A1143 for example.

If everything looks good, remember that the Time Capsule will need to be located where it can receive a very strong signal from your main Apple router. It won't work if it is buried in a cabinet or the wireless signal has to travel through 4-5 walls to reach the Time Capsule

It will work, but everything that is connected to the Ethernet ports on the Time Capsule will have to depend on the quality of the wireless "hop" between the AirPort Express and Time Capsule. Frankly, that makes me nervous.

It would be much better if you could connect the AirPort Express and Time Capsule using a wired Ethernet connection.

Keep that wired setup in mind as a Plan B if wireless is not delivering the performance that you need.

This is going to be one of those situations where you won't know if wireless will work for your pupose until you try.

Good luck, keep us posted.