If you simply want to write say a python program and put it on a USB memory stick then it should be possible to instruct the Raspberry Pi to check for that being inserted and run any programs placed on there.
But as said above it can't be used as a USB device directly.
The closest you'd get is using a direct network cable link and using X-11 forwarding you'd have access to do most things. Other than the link being Network rather than USB, it would be close.
http://pihw.wordpress.com/guides/direct ... onnection/