There is no direct way, but you might be able to accomplish it through other means. For example, if you have a friend who wants to take a flight and will be BUYING a ticket, have them give YOU the money and you spend your 25K miles to get them the ticket they were going to buy anyway (or something comparable). Then you've "spent" your miles and gotten cash that you can use to buy whatever you want. You could accomplish the same thing via eBay, but would probably get less money out of it that way.