Yes—this is usually possible in post-processing, though the exact level of automation depends on the software.

From the answers given:

• Nikon ViewNX-i can copy geotag information from one photo to others.
• Lightroom can do this manually: load both the iPhone and DSLR images, find the iPhone image on the Map module, then drag the related DSLR images onto that location.

So the practical workflow is to import both sets of images, use the geotagged iPhone shots as references, and then copy or assign their GPS metadata to the Nikon files taken at about the same time/place.

If you want something more automated in the future, one answer recommends using a GPS logging/app workflow instead of relying on nearby phone photos. That can be easier than using a separate camera GPS accessory.

Key things to check:

• Your camera and phone clocks should be reasonably accurate.
• Your software must be able to read the iPhone’s location metadata and write GPS data to your Nikon files or sidecars.