So, me and my friend had this idea: you take a grenade or some type of targeting device, when the grenade goes off or you target an area, a dropship of your choice(editable outside of the game) will fly in with a certain amount of bombs, of your choice , and fly around the area dropping the bombs. Would this be possible? Has it already been done?

It's been done to an extent. Look under minor mods for a sort of drop pod thing. Throw a grenade, it signals a craft to come down and release soldiers. the Doomsday mod might also be of some use to you.