I think you may be a bit confused. Wonder trades are where you offer any Pokemon, except special Pokemon and items like event Pokemon such as genesect and mega-stones and z-crystals, and get paired with a random person who has also offered a Pokemon and you do not know what the Pokemon that you are going to get will be, hence the name wonder trade.
Now wonder cards are basically a proof of purchase in a way because you get them with a mystery gift and have a set number of wonder cards that you can receive, if you ever run out of room you can delete some be be sure to pick up your mystery gift first.
Now to answer your question, no you were never able to trade wonder cards and z-crystals function in the way as key items do meaning that you can not trade them (though I think that if you and the person you are trading with both have the z-crystal being held by the Pokemon you can trade though I am not entirely sure and it would not help you anyway) and if you missed am event for a z-crystal it is unfortunately impossible for you to get it unless they send it out in another event again, I missed the mewnium-z distribution also. So to sum it up, they never used this feature and they probably will not use it either. Hope this helps!