When in Extend Mode, can I use my AirPort Extreme's WAN port as a LAN port?
Yes.

When an AirPort Extreme is configured to extend another AirPort Extreme wirelessly, the WAN port is not used for WAN, and this means it is automatically switched to be a LAN port, identical to the other three LAN ports already on the back of the unit. This was tested by myself with an Apple AirPort Extreme 802.11n 1st Generation, and this answer was posted with a computer which is connected to the WAN port of said router.