It’s very hard to get redemption and revenue tickets on the same PNR, even with the same airline/FFP program. There is certainly no online way to do it, if any way at all.

However, to jump out of the box a bit… unless OP is flying OW from DTW to LAX, CX is not obligated to protect in case of IRROPS, even if the tickets are under the same PNR. And IIRC neither AA nor AS flies DTW-LAX, so that would mean an extra connection. Is it really worth it then? (over DTW-HND/ICN - whatever?)

If I were based in DTW I would probably prefer BOS/JFK or even YYZ to the west coast departures. CX843/5/811 leave very late so unless there are major disruptions or cancellation, it will probably be fine.