Airport Extreme "Snow" to "Extend Network" (rather than WDS)

Can I use "extend" rather than "WDS" to increase wireless coverage on the "Snow Dome" Wireless G version of airport Extreme?

No, the "extend" feature was introduced in 802.11"n" routers. You have an older 802.11 "b/g" device which is not compatible with the "extend" feature.

I want to extend my ATT U-Verse wireless network.

ATT U-Verse routers are not compatible with Apple products if you are trying to "extend" using wireless only.

I see this option for the new airport Extreme base stations that have 802.11 N.

Correct

I DON'T see this option for the new airport Extreme base stations that have 802.11 N.

I'm guessing that you meant to say.........Extreme base stations that are 802.11"b/g" only....and not N as you mentioned. As mentioned above, this older version of the Apple router does not have a setting to "Extend a wireless network"

I want to extend my friend's wireless network (Non-ATT / unknown smaller provider)

Apple's "extend" feature will only work with other Apple 802.11n routers, if you are trying to "extend" using wireless only.