Global Entry isn't airline specific. Anybody can apply and go through the process to get cleared into Global Entry. Whether it is offered at airports is left up to that of Customes and Immigration. I do believe, however, that Global Entry is available at every airport that sees international traffic. Including SJU. I know 100% that it is offered for you at SJU. It's at the far right had part of the immigration area next to the crew and diplomat lines