The only way to trade pokemon that aren't in your pokedex is in gts negotiations then you can get any pokemon as long as it isn't an event pokemon. But on the gts in the other hand, it will only let you select the pokemon you want to trade if that pokemon is on your pokedex, but if an event pokemon is on your pokedex then you can get it on the gts.
Hope this helps ;)
-Green Scizor