I doubt it. The tech specs page lists the following:
"AirPort Extreme, Wi-Fi 802.11b, 802.11g, or 802.11n wireless network, or 10/100BASE-T Ethernet network"
Its an OR condition, not an AND condition.
Out of the box, its either wireless or wired. It can't be both. But as mentioned previously, perhaps hackable to do what you want.
From using mine, it is always a client. It is not a server/router/NAT-capable entity.