There is no user-friendly way to add emulators or cores to Batocera. This is by designed, the emulators are compiled and patched specifically to function on Batocera (which is a “Just Enough Linux” OS, not every application expects that and some need certain “fixes” to still work, RetroArch cores are no exception).
If you want to build your own image with your own cores enabled, you can learn do so here: https://wiki.batocera.org/compile_batocera.linux
I’ve tested this myself. Don’t. Save yourself the time, please.