You can't apply for an extension of stay for your visa at the China embassy in KL, once the visa is issued, you can't change it. You can go to China first by using this visa, then go to the local exit & enter department under Public Security Bureau to apply for extension of your visa before 1 week when the stay duration of your visa will expire. In fact, you will be issued a new visa on your passport to permit you stay for another days in China.