I don't think it's possible to search for this automatically. However, you can work it out by looking at the fare-rules. The example you give is for an open-jaws ticket - and in particular, a single open-jaws (also destination open-jaws), as it's only 'open' at one end.

So, first you look up the fare rules for a return YYZ-HAN, and see what it says under 'combinations'. In many cases, it will say something like:
'Open Jaws/Round Trips/Circle Trips - Fares May Be Combined On A Half Round Trip Basis'

You then look up the fare rules for a return YYZ-SIN, and hopefully it will say something similar. If so, then a single open-jaws for that routing is possible - and the fare is calculated by adding half the YYZ-HAN return fare, and half the YYZ-SIN return fare. As you say, a return is often cheaper than two one-ways - and an open-jaws basically gives you the average of two return fares, and so will often also be cheaper than two one-ways.

You can also do departure open-jaws (when you return to a different airport from the one you flew out of) and double open-jaws (eg: AAA-BBB, and then CCC-DDD). However, although single open-jaws often seem to be possible, double open-jaws, or departure open-jaws (especially if returning to a different country) can be harder to find. There are often some restrictions imposed on open-jaws routings (eg: limiting the distance between the two 'open' points, or requiring them to be in the same area).

If you wanted to do, say, YYZ-HKG, HKG-HAN, SIN-YYZ, then that's a stopover in HKG (plus an open-jaws). Whether that is possible will depend on:
a) the routing rules (ie: is travel via HKG allowed), and:
b) the fare rules (ie: are stopovers allowed, and if so, are they free or do they require an additional fee)

I regularly travel on such tickets (although not on the routing or airline you suggested) so it's definitely possible. However, the biggest problem often seems to be that although open-jaws and stopovers are allowed, online booking sites (even the airline's own websites) struggle to book anything other than simple returns.