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. 

This is possible if you do not have a suitable adapter ring. However, care must be taken when reducing larger lens diameters because of possible vignetting.  

This is possible if you don't have a suitable adapter ring. However, when reducing larger lens diameters, caution is required because of possible vignetting. 

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!