They seem to be just a regular 52mm circular filters mounted onto a sliding frame.
I'm not sure if the existing filter is removable or not, so I'm wondering—
1) Should I remove the existing filter off the frame and then use whatever filter I want? OR 2) Is the filter non-removable, so I should buy more drop in filters? The selection seems quite limited? OR 3) Does Nikon make a filter holder I can't seem to find anywhere? (I would assume this part is very expensive as well)