Our flight only has a 15 hr stopover in Beijing, can we extend this to a 72hr layover?

Hi, we are confused! We want to book a flight from Bangkok to London. The scheduling suggests we would have a 15 hr stop in Beijing. We would like to extend this to take advantage of the 72 hr allowance. Can we do this on this ticket, and just get on the next available flight (sorting it out on arrival in Beijing), or do we need to book two separate flights, one from Bangkok to Beijing, and another from Beijing to London 3 days later?

Thanks!

Joe