I was told by the AA staff that the only way to do this is to have the FF number changed after you get the boarding pass at the airport. . .but, on my last flight from NRT to SEA via DFW, I asked to have the DFW to SEA leg credited to my Alaska account. Not only did they do this but my boarding pass for that segment showed my status with AA even with the miles going to AS. I still wonder how they did that.