Use complex collision as simple, But with lower LOD?

Hey.

So Im creating the collision for a rock that has lots of convex and concave shapes. Is it possible to use complex collision as simple, but with a lower LOD to make it more optimal? Also is it possible to import my own collision to be use in this way, allowing me to have concave mesh collision? Why does “use complex collision as simple” allow for concave shapes?

Thanks a lot.