I'm pretty sure you can't. The only way I would think it possible is if they made some sort of memory card adapter like with the ps2. You know so that you could attach an old xbox memory card with game saves to an xbox 360 and then transfer the saves.
I was curious about that as well. I'm still waiting for my hdd but I was thinking about transferring the game saves to my PC via action replay or just straight FTP (my old xbox is modded) and then transfer the game save files over to the 360. I'll give it a try this week once I get my hdd but any insight in the mean time is certainly welcome. Thanks