You could do that. Now, one would not consider SIN as a stopover as that should, I assume, be your destination if you choose to fly Malaysian. That being such, you could do a stopover in Kuala Lumpur, where usually there would not be a significant price difference.
To book such a fare, do use the Multi-city option when you book your tickets.
Although, do bear in mind that the price differences may vary by date and seasonal demand, so do also look for other airlines as well, as there is a great likelihood that they may have cheaper fares.