Yes you can eat hailstones or even use the hailstones as ice in a drink.
The hailstones are just actually frozen water just like ice that comes out of an ice machine the sky naturally makes that ice.
I've used hailstones in my drinks to keep them cool and I've also bagged up some hailstones and used them as ice in my cooler to keep my beer cold.
The hailstones are basically just natural ice and there's nothing harmful or wrong about eating them or using the hailstones as ice.
Just make sure you get the hailstones from clean place and as long as the hailstones are clean they are safe to eat.