The adapters that convert a USB keyboard to a PS/2 socket are dumb adapters and rely on the keyboard's electronics doing the PS/2 magic. I very much doubt they would work with an ADB keyboard converted to USB.

On the other hand, if you have a USB socket on your computer, you could try mounting your USB-ADB keyboard directly

Kbdbabel has moved along since it was first cited here, but it still doesn't support ADB to PS/2

Here's a cache image of a DIY ADB-USB converter