def METHOD_NAME(self):
    build = self.get_finalized_command("build")
    missing_cmds = set(build.get_sub_commands()) - _ORIGINAL_SUBCOMMANDS
    # ^-- the original built-in sub-commands are already handled by default.
    cmds = (self.get_finalized_command(c) for c in missing_cmds)
    files = (c.get_source_files() for c in cmds if hasattr(c, "get_source_files"))
    self.filelist.extend(chain.from_iterable(files))