You add them to the filesystem already on the PC. One of the final steps of Buildroot before building the filesystem images is to call a so-called final script. This script can be used to copy some own files to the folder output/target. Then these files will be part of the root filesystem images.
F&S already uses such a final script to copy some of our own files to the directory. These files are located in board/f+s. The name of the script is available in Buildroot's menuconfig. You can either add your own script (the entry in menuconfig can be a list of scripts, seperated by blanks). Or you modify our script and add some lines to copy your additional files.
Your F&S Support Team