How to aproach to ADCOM guys?

Hi. 640 is a decent score and as you know, you should try to get something more interesting, closer to the magic 700 number.
The good news is that I have been seeing a lot of South American candidates get admitted with 680 scores (to top 20 and top 30 programs, places where Asian applicants could not touch with anything less than 730).

However, I do not think you really need to talk to the adcom about anything at the moment. You are definitely welcome to get in touch and signal you are serious but in general the application system works to process you without having to speak to anyone In any case, 640 is probably below average for all of those schools and I would wait until you have a chance to retake before you start chatting them up.

P.S. There is a debate about school visits - if you have an opportunity to visit the schools in the fall and potentially attend and sit in one of the classes, this would be a big plus both for your personal information, your school information you can use on the essays, and also adcom keeps track of those who visit and considers those visiting more committed applicants, esp international ones.