From https://docs.godotengine.org/en/stable/classes/class_diraccess.html
func _import(source_file, save_path, options, platform_variants, gen_files):
var file = FileAccess.open(source_file, FileAccess.READ)
if file == null:
return FAILED
var mesh = ArrayMesh.new()
# Fill the Mesh with data read in "file", left as an exercise to the reader.
var filename = save_path + "." + _get_save_extension()
return ResourceSaver.save(mesh, filename)
"left as an exercise to the reader" 😆 I love it!