Why not? The market is flooded (or was...) with USB to PS/2 adaptors that came with mice & men keyboards.It is, however, way outside the scope of this.
'Sides, there goes the question...why not? What is not known about the USB standard? We (might) have the working elements of a ADB controller, so a simple microcontroller (ahem Arduino? nah that's probably way over, but if it wasn't done with an Arduino than it wasn't proper) with the code and a small etched or prefabbed circuit board and *KABAM!* we now have not just PS/2 but many other things that could be useful: like USB optical trackballs.