Hmmm. I just tested that. You can give a unit that does not have an attack - like an albatross - attacks stats, but in-game, it will not attack.
I tried it in this map.
The albatross has been given attack stats, but it cannot attack in-game. I dunno, see if you can make the albatross attack.
Back on topic, if you want to give a unit an attack but is doesn't work for some reason, you can just take a unit that can attack and change it's model.