Knowing nothing about it yet, my guess is that you'll need to run a PC version as a "bridge".
Let me explain a little.
When the XBox360 is connected to a network, and that same network has a computer connected, a bubble-window thingy appears in the bottom-right corner of the screen, next to the network status, saying that a Windows media extender is available.
By registering the 360 with the computer you can (I believe) download files between the computer and the 360.
If nothing else, we could save the sims as a video file on the 360 (a corrupt one), and transfer them to the computer, changing the extension to the correct .xml (I assume that is what it will be) extension.
I am 90% sure you can transfer video files, which is why I chose that as the example format in this little rant
