I've done this for UA flights. You just need to call the service center (or your preferred line) and tell the agent that you'd like to cash in your miles for a ticket on AC. In my case, I said I wanted to fly on UA, the agent typed away for a bit, and then confirmed the flights with me. She seemed a little put off at first that I was requesting another airline, but got me what I wanted.
I'd imagine things would work the same for a flight on AC. Let us know how it turns out...