I was pretty excited when I first saw the designs for the new Hallow's End witch hats. In spite of them being marked as cosmetic items, it turns out that they are not transmoggable. Blizzard themselves have confirmed that this is intended in the following tweet (source):
Originally Posted by BlizzardCS
We once had this problem with the Crown of the Fire Festival, and I urge everyone who is let down by this fact to voice their dissatisfaction on the official forums or through the in-game suggestions form as it seems to have helped before.