Yes, you can transfer Amex MR to Mexicana (1,000 MR: 1,600 Mexicana km), but now you're getting into an opportunity cost issue. There are many other things you could do with those Amex miles, whereas you can't get 1,000 miles for a calling card with other airlines.
Most people here will be able to get the Mexicana miles through HHonors or a hotel stay. See:
http://www.flyertalk.com/forum/Forum1/HTML/003547.html
I suppose for someone who wants to purchase nothing, you could do the following:
1. Update your HHonors account twice a quarter (2,000 points) until you get 10,000 HH points.
2. Transfer 10,000 HH points for 1,500 Mexicana miles.
3. Transfer the 6,250 Mexicana back to Hilton for 20,000 HH points.
Net result: For just opening a FF account and updating your HH account, you net 20,000 HHonors points, or 3,500 air miles on a major carrier.