Normally you can use this link on webflyer.com:

http://www.webflyer.com/programs/mileage_converter/

But for some reason Delta does not appear in the "convert from" column, only the "convert to" column. I do not know of any way to convert them that will result in a decent return. The only thing I can think of that MAY work is if you have a Diners Club card and Diners lets you transfer Delta into Diners (they allow some miles to go into Diners like AA, not sure on DL) you could then transfer them out to AA. You would suffer a 50% devaluation. I am sure that there are some other convolluted transfers that could be done, but many of them would devalue so much that they are not worth it at all.