Sorry, but I am almost certain it cannot be retroactive. Go here for the details:

http://members.shaw.ca/fewmiles/AA/index2.html

I called AA 800.882.8880 and asked to be connected to AA CS and then said I wanted to sign up for the GLD and PLT challenges at the same time. It was done in less than 30 seconds!

You can call now to sign up as of March 1 or whatever future date you want...