Actually you can reverse transfer, it is very hard to do though, and you may have to argue special circumstances such as I did.

American Express can't initiate it. I transferred points from American Express, it was about 80,000 into Continental, then booked a pair of 1st class tickets to Maui. Continental changed the flight on me, I called them very upset, as the new flight was much much worse than what I had booked. 3 segments vs. 2, and 1 leg not being 1st class (to small of a plane). 1st call did me very little, so then I called AMEX, they told me that Continental could do the reverse transfer, two calls later I ended up transferring 150,000 miles to American Express, then transferring them into Delta and booked award tickets that way.

It was a process for it to happen, 3 calls to Continental to get it done, but worth it in the end for me.