AA will not give you credit if you were not a member at the date of the flights. This policy is comon with many ffp's
US Airways does/(did?) allow are credit for flights taken up to 6 months before you join.
You are better to join US and retro claim for the LAN/AA flights
Some information on retro credit in post 1 here
http://www.flyertalk.com/forum/infor...help-here.html
And here
http://www.flyertalk.com/forum/22311026-post3.html