Is it possible to install a main drain in an above-ground pool?

It might be possible, but not easy, and you may be left to invent your own drain fitting. Draining and above-ground pool is generally accomplished fairly readily by siphoning or pumping through a vacuum hose. Yes it's possible. You can use a hayward main drain. I think it's an SP-1048 or SP-1408. Just make sure you set it in a small amount of concrete under the sand surface to prevent it from sinking.