Once CO joins *A you can use onepass miles to redeem on *A airlines. However the miles aren't transferred to another program to do so. It would be a CO award on their rules and at the mileage cost level they set.

I expect it will be a while before CO joins *A. Thus if your travel is within the next 1-2 years you may need to transfer from Amex to a different program (one that is already in *A).