Can I use Nikon Capture NX-D when a RAW is stored in Photo's?

Export it as an unmodified original.

Which is why you want "Export Unmodified", not export.

Export photos, videos, and slideshows from Photos on Mac - Apple Support

As keith said - you can export the unmodified original.

A question though is why you want to. You can process the raw file (for compatible Nikon cameras) directly in apple photos. Or does NX-D offer something that photos cannot do?

While I agree with you, often the Mfg's software can do things that Photos - or maybe even adobe - cannot.

For example:

Honor the black/white setting on the camera and display the RAW in B&W

Display the image exactly the way the camera's jpeg image would do it, with the cameras sharpening and color styles.