Yes, this type of scenario is fine.

You'll be paying the higher mileage amount, though. Flights to Moscow are 40,000 off-peak and flights to China are 50,000 - so expect a 50,000 quote from AA.

I know this works, because I've done a similar trip myself:

AA into Brazil, then out from Punta Arenas, Chile.

Mine were in the same zone, though, so I paid one price - 40 K. But, of course they would love to give you a lesser worth ticket, US-Moscow as opposed to US-Asia.