Dear Tim
Dear Kris,
As far as the airline programs go, only American and US Airways allow members to use each other's miles. And it's not quite an outright transfer, either. Someone who is a member of both American's AAdvantage program and US Airways' Dividend Miles program can, for example, transfer enough AAdvantage miles into his Dividend Miles account to qualify for an award on American or US Airways. (Or vice versa… the American-US Airways relationship is reciprocal.) Note that the miles can only be combined at the time of redemption, and that they cannot be used for awards on all airlines in the program--only on AA and US flights.
There's another way of exchanging miles: Hilton HHonors' Reward Exchange feature. Reward Exchange allows HHonors members to convert miles from 11 airlines into HHonors points, and from HHonors points back into miles in 11 airline programs. It's wonderful in theory, but you pay a price for the flexibility. If you started with 10,000 Delta miles, they could be converted into 20,000 HHonors points, which could be converted again into 3,500 Continental miles. So in the conversion process, your original miles have lost 65 percent of their value.
Hope this helps.
Tim Winship Contributing Editor SmarterTravel.com