Currently, it seems that mostly they are under their respective system if it's the only case for the disc, or under Swag > Carrying Cases & Bags if they're an additional bonus on top of the other, normal game case. I personally think that it's a good way to separate them and would continue with that logic.
Tl;dr:
- If it's the sole case to your game, enter the game as Steelbook Edition.
- If it's another case additionally to the normal game case, enter the steelbook separately under Swag > Carrying Cases & Bags (since they're cases).