Haven't tried this but will probably work in a pinch.

Pap




http://www.dump.com/sharpenyour/

I've done this with a Case folder when I worked in an office years ago. It worked okay, better than nothing. I think a lot depends on the cup being used.

The Case was stainless. Probably carbon steel would work better.

My grandfather didn't believe in high-tech-equipment. He used to sharpen his knives on the back of a plate - it did work very well