You mean how to merge HEX files? there is mergehex tool in nRFTOOLs bundle (next to nrfjprog utility). Or simply copy and paste parts of the files as these are plain text.
When it comes to "mass production" it largely depends on your set-up. Typically your CM will already have some standard tools and processes. In case you design entire production jig then you can take any of recommended solutions from Infocenter or search for other options on blog zone.
