Most (if not airlines) have corporate accounts, which allow you to collect the miles as well as get a discounted tickets. However, I doubt if they will allow small charity to use those programs. There are spending expectation, and they are geared toward medium-large business.

You might also look at pre-paid accounts that several airlines have. In essence you buy upfront a "credit" on the airline that can be used latter on. I'm not sure though, what are the rules of who can use it, and if miles can be deposited to a central account.