Compatibility of USB keyboards varies greatly.
The BASIC Engine only really supports PS/2 keyboards. Many USB keyboards have a PS/2 compatibility mode and switch to it when they detect PS/2-like signalling on the data pins, and if you connect such a keyboard to the "USB" ports on the BASIC Engine, it will be used like a PS/2 keyboard.
I had originally assumed that most, if not all, USB keyboards are PS/2-compatible, but that does not seem to be the case; having gone through a pile of old keyboards I bought in bulk on eBay, I found that the rate is more like 50%...
Some signs to look for: If a keyboard comes with a USB-PS/2 converter, it will work. If it has additional USB ports, it most likely won't. In all other cases, trial and error is the best option.
Note that PS/2 keyboards will always work. If they don't, it's a (software) bug.