There has already been add-on digital backs for SLRs, even one from Leica. Those times passed 10 years ago.
Nikon is not patenting a digital back, but some adjustment systems for precise sensor placement in existing 135 film cameras.
I doubt if anything comes out of this in the end. Back alone would end up costing more than a decent DSLR and would be clumsy and heavy. Not worth it, but patenting things is comparatively cheap.