batocera is like a firmware (it's mainly a single file).
it comes as it is. you cannot modify the system (ok, there are some tips).
to add an emulator, you've to add it in the build process.
for example, to add dolphin (wii/gamecube), i had to write this (and a bit more) : https://github.com/nadenislamarre/batocera.linux/tree/master/package/batocera/dolphin-emu
yes, the aim is to be able to replay a maximum of old system. but while the number of players is very high, the number of contributors is very low, so, it takes time. but you can list here which one are missing (and i've several others in my todo list to add too). and better, you can even contribute ;-)