Yes, drum sanders are similar to planers. Both register the board on the bottom surface, which is pressed down by the feed rollers, so the top surface will end up following the profile of the bottom. By using a sled of flat wood (i.e. plywood), and shimming under the high spots on the bottom so that the feed rollers cannot deform the wood you can get a flat surface on the top. When flipped over and run through the sander/planer, this flat surface on the bottom will result in the other surface being flat as well. Lots of info on the web about making/using a sled.