yes you can edit es_systems.cfg , on intenet you will see alot of pack i created my self .. like kof collection zelda collection ,.
you are able to mix system for example all sony systems in one or nintendo system all in one .
that requires alot of time cause is not only the es_systems.cfg you need to config.
#after you need to create a repective theme for that collection .
#you need put all roms together from you collection .
#you need to config every single games on batocera for avaliable cores on that collection .
For example if you created a collection with ps1,psp and nintendo roms, you need to configure rom by rom and add the correct core to work for every single roms .
The scrap you need to create manualy cause scrap programs wont detect remixed systems .
es_systems.cfg need be located on recalbox/system/emulationstation