Yes, China though may not have been my best example as China is not covered by the JV, meaning there are some purely NH fares that are cheaper than UA that this won't work those. However nearly all native UA fares work. (Perhaps some G fares are excluded but I don't think I've never found anything L or above that didn't allow stopovers). Expertflyer is a great tool to assistant in finding these.

Just search as if you were buying a round trip to that point to start (PVG, HKG, BKK, etc), then once you find a price you're satisfied with, try using multi-city to force a stop in TYO (use TYO in your search instead of the airport as it'll give you both NRT and HND arrivals and departures. As long as the fare class is available both to Tokyo and the NH flight from Tokyo to the final city then it'll only go up $100 and a few dollars for Japan tax. I'm nearly certain the stay in Tokyo will apply to minimum stay requirements, allowing same day turns in the final city.

Assuming they have a US/Canadian passport or one of the dozens of others allowing TWOV, then they don't need a visa. The flight would be TYO-PVG(or PEK)-USA/Canada without returning to Japan first. They only need a visa if they transit Japan again on the return, but HKG and Korea are fair game.