That's a little different, aa.com is not very flexible when one is after a particular fare class and nothing else will do. The best you can do is price it at the lower class and get the agent to upgrade the PNR by hand, and pay the difference, naturally.

However, before you do further work on this,... are you aware that:

1. You cannot upgrade with miles an EZE-EUR through fare. You would have to upgrade EZE-USA + USA-EUR, at double the cost in miles.

2. Not all B classes are created equal. The B class fare you are after, even if you were able to get it, may not be upgradeable with miles at the reduced level if it is a restricted B fare. (edited to allow for exceptions)