In order to extend the Fios network correctly, connect an Ethernet cable from one of the LAN <-> ports on the Fios router to the WAN "O" port on the AirPort Extreme.
Insure that the following settings are used when you set up the AirPort Extreme:
Wireless Mode = Create a wireless network
Wireless Network Name = Same name as the Fios network
Wireless Security = Same security that the Fios network is using
Wireless Password = Same password that the Fios network is using
The tricky part of this for most users is the the Wireless Security setting, since most users know the name of the wirleess network and password. But....if the Wireless Security settings do not match up on both the Fios and AirPort routers, the network will not operate correctly. Hopefully, the Fios router is using good security like WPA2 Personal.
The final....and very important step...is to insure that the AirPort Extreme is setup to operate in Bridge Mode.
Concentrate first on getting the AirPort Extreme working correctly, then we can discuss using an AirPort Express to extend the network further.