The rules don't explicitly say if you can or cannot look at the card that you're adding from the horde deck to the turn deck, before reshuffling the turn deck.
You cannot look at the card being added.
If you cannot, that means you won't know where the enemy will pop up until you encounter it for the first time?
Correct. This is a large part of the game's challenge. You don't know for sure what might be coming or when.