There is no way to transfer Amex Rewards to AA or UA without major devaluation of your points.

But there is an alternative way to approach this situation. First, open a Delta Skymiles Account and a Mexicana Airlines Frecuenta account. Second, accumulate enough points on Amex for a desired reward. Then call Delta Skymiles and/or Mexicana Airlines and arranged for a reward reservation. Finally transfer the Amex miles to the desired airline Delta or Mexicana. Note Delta's agreement with United ends soon, so act quickly. Good Luck!!!!1