I just checked the AirAsia website and I see what you mean there Jacketwatch. Unfortunately, multi-city is only available on AirAsia's long-haul arm, AirAsia X. As such, AAX does not fly to Singapore, but it's main operations does.
AirAsia, being a low-cost carrier saves loads of money by limiting this as it would mean giving the liability to the customer to handle one's luggage and check-in for the short-haul leg.
Being so, if one really want to save and choose to take AirAsia, it may be a good idea to allow 3-4 hours between the long-haul and short-haul to clear immigration, reclaim luggage, check-in and etc.
If one considers it not worth the hassle, then I'd most definitely take a full-service carrier like Malaysian or Singapore Airlines for the mentioned destinations.
Otherwise, one could definitely book three one way segments (i.e. PEK-KUL-SIN, SIN-KUL, KUL-PEK), which should probably be the same price considering that this is on a low cost carrier.