I am not the creator of this taunt. However, it's probable that this taunt moves the hat bone, rather than the Engineer's hard hat itself, therefore any hat you are wearing will work with this taunt.

k thanks for the explanation

It won't turn any other hat into a hardhat - if this gets in, theoretically valve will simply add hats onto the same bone as the hardhat and hats would rise, but other cosmetics wouldn't.