If I fly from UK to Beijing for up to 72 hours, can I get the same on the return journey?

Hello, I'm planning to fly to Beijing for 2/3 days then onto Seoul for a week then return back. Is it possible to get another few days in the city on return from Seoul? Also these would technically be separate as Air China only does 'short' layover times but I would be in transit. Would it need to be visible that I am in transit on ticket or would this method be invalid?

Graham