In practice, no. A Nikon Nikkormat was designed for 35mm film, and a true digital conversion would require fitting a sensor, power, image processing, storage, and controls into a body that was never built for them. There have been occasional niche or experimental ideas around “digital film” modules, but they are not a practical, mainstream solution for old film SLRs.

If you want to keep using the camera, the realistic option is to shoot film and then scan the negatives. If your goal is to use the lenses, many Nikon F-mount lenses can be adapted or used on digital Nikon bodies, depending on the specific lens and camera. That usually makes far more sense than trying to convert the Nikkormat itself.

So: keep the camera as a film body or display piece, and consider a digital body separately if you want digital capture.