I own USB Host to PS/2 Keyboard/Mouse adapters. Not sure if they are passive, but they do work and aren't terribly expensive. I mean you could always replace the keyboard controller with a USB one, but that is a bit overkill, and for the time you spend you could buy a really nice USB keyboard with MX Blues.
I have NEVER in my life seen a PS/2 host to USB adapter. I GUESS I see how it could be possible, but I see no reason for one as most machines with PS/2 have USB support, even if it's through a card.