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Why wouldn't you be able to fly DTW-EWR as part of your EWR-PEK award for no additional mileage?
Just check united.com for availability on all Star Alliance carriers. Any itinerary that comes up as "Saver" should be bookable with your US miles by giving US a call.
I'm not sure if US would allow a TATL routing for DTW-PEK, but if that is allowed, then DTW-FRA-PEK on LH might be another option.