Usually, no standard front screw-in filters can be fitted to ultra-wide or fisheye lenses with a bulbous front element and fixed petal hood. The main issues are the lack of filter threads and the extremely wide angle of view, which makes normal-sized filters vignette.

If you want to use ND filters, the typical solution is a specialized third-party oversized filter holder system made for these lenses. Community examples mentioned include Fotodiox Pro adapters and Lucroit/Hitech systems. These use very large filters placed far enough forward to avoid clipping the frame.

Some lenses also use rear or drop-in filters, but that depends on the lens design and is not standard for these models.

So the practical answer is: you generally cannot use ordinary front filters, but you may be able to use a dedicated large-format adapter/holder system designed specifically for bulbous ultra-wide lenses.