This filter screws into the lens. 

The front lens of the lens is strongly curved and has a permanently integrated lens hood, threads for screwing are not available on the optics. The cap is placed over the lens hood and held in place by latches. I don't see any options for attaching a filter, and I'm not aware of any factory solutions or products from other manufacturers. 

The Answer is Yes. You can use a pinch cap over most filters. ALL my lenses have a filter of some sort on them. I use to use a lot of "traditional" lens caps on all my old Nikkor lenses but putting them on and taking them off became a giant PITA, so I got a Pinch Cap as a test. I then converted ALL the lenses to Pinch Caps. Hope this helps. 

I've never used Hoya ND's, but I have GOBE, KERNEL, ZOMEI, and K&F ND filters. Out of all of them, I like the K&F the most because the colors look great out of the camera and they don't create a vignette around the edges like some of my other nd's which I hate. Because of the great out of camera colors, it gives you more to work with on post-production when you shoot on a flat profile. Also, I've used them during night photography on bulb mode to do light painting and the 4 - 6 stop ones work great for this. Overall, I think for the money the K&F filters are worth it. Awesome product! 

Hi, the K&F black soft filter 1/4 has the same effect as Tiffen black pro-mist filter 1/4, just difference in name , thanks.