You can't make a hole in the ground, but nothing says you can't make things that go above the ground. Basically, you could have a crater, where the very bottom is at the ground level, and the walls are above ground. Craters do that somewhat anyway.
Just look at a "trench" addon, or that tank-ditch addon, and you'll understand what I'm talking about.
This is a good idea though, and i would love to see done by someone who can use O2. I'd be curious to see if we could replace the flat OFP craters with this kind.