Can I use a USB keyboard with the PS2Keyboard library?
This question is a little bit broad to answer because it depends entirely on the keyboard itself.

If the keyboard has the hardware to use the PS/2 protocol, this yes, you might be able to use it with an adapter. If, however, the keyboard only has the hardware for the USB protocol, then your adapter must do it.  

Most of those USB to PS/2 adapters are just physical connectors.  It was up to the keyboard's hardware to use the right protocol.  

Electrically USB and PS/2 are vastly different protocols.  In terms of protocol implementation PS/2 is a simple clocking scheme while USB is a fully layered protocol stack.

As for forcing the keyboard to speak one or the other, that's going to be ENTIRELY dependent on the keyboard.  I doubt there's any kind of reference which would provide that kind of information.