How can I add filters on Tokina 16-28 f2.8 Lens?

[killswitch]

I am looking to purchase the Tokina 16-28 f2.8 . Due to its bulbous front element there is no filter thread. Are there any adapter/method or some sort of holder out there that are used on such designs so that one can use filters. I am hoping to use ND filters and really need to figure out there are ways I can attach filters before I hit the buy button.

Thanks