If you have a confirmed and ticketed reservation with some less desirable times, they can wait list your more preferred dates for you. But they are not allowed to hold confirmed seats for you to wait for the wait list to clear and ticket. (like for award tickets)

That being said, you can purchase a less desirable ex-China flight now and do a wait list for your preferred ex-China flight. Be aware of date/time/min-max stay requirements though. When you change your first segment, YQ and taxes needs to be recalculated but fare usually stays the same (refer to fare rules)