you need to know how to reqrite modules and codes is not anything simple to do to add kirby hatcs that is why brawlex doesnt use kirby hats and then you would need to psa the kirby hat under kirby and the filesize would increase so its definitly not anything that is easy to do unless your a master coder and know everything that isnt public about rels which barely anyone even knows how to edit.
kirby has no real file size limit, so its fine
but to answer the orignal question, they have a code for the roy and mewtwo kirby hats that loads the new files.