Converting raw images

I set my camera to take raw images, I have a Nikon so all the images are of .NEF file format. I can convert it to JPEG and PNG with my Mac but now I need the NEF format again to do some editing. But when I convert it back to NEF (with ImageMagick) Photoshop tells me that it is a filetype that it cannot read, even if the NEF extension is there.

Is there any tool I can use to get back my NEF file?