Often a stopover of 24 hours will be allowed on an international itinerary, that does not seem to fit what you want. Those can be difficult to force anyway since the airline computers assume you want short, not overnight, layovers.

You need to do the legwork to get the best price. My preferred site for this is matrix.itasoftware.com. Google Flights is also good.

1. Price out a round trip DTW-China that stops in SFO and contains a < 24 hour connection. Note that if there are irregular operations the airline is under no obligation to route you through SFO if you go this way.
2. Price out a multi-city DTW-SFO, SFO-China, and China-DTW routing for the dates you want to fly.
3. Price out one way flights DTW-SFO, SFO-China, and China-DTW. Also check Southwest.com for the DTW-SFO leg as they do not participate in most consolidated booking engines.
4. Price out round trips of DTW-SFO-DTW and SFO-China-SFO including Southwest again on the domestic leg.

There are more possibilities, but that gives you the general idea.