There are tools like Exiv2 that in principle allow you to read from and write to the metadata in image files, even in raw files like .NEF files. But you'll have to figure out which tags to write to, and to my knowledge, EXIF only allows ASCII characters (7-bit).

Do note that some consider writing to raw files as bad practice. Raw file formats are usually not well described. This means that it's easy to make an error that makes the raw file unreadable. And the raw file is more or less your base data (like the negative in the film days). That is also the reason those sidecar .XMP files are created by most programs that modify the metadata.

An exception to this is the DNG format, as long as it's not your camera's native format: while this is technically a raw format (in most cases, there is also something called lossy DNG...), the DNG format is well described. Also, .DNG files are usually created from another raw format, and as such are not the original data. (again, unless the camera creates the .DNG, which is an option with certain Pentax models).