I don't think DOS can use USB (including USB keyboards) because it would need a driver (are DOS USB drivers available?) to be loaded after the operating system has started. The catch 22 is that DOS won't load without it recognising a keyboard is attached. During its boot process it looks for certain devices including keyboard & monitor....but until it's running & loaded its device drivers then it can't 'see' a USB keyboard.
Since DOS looks to the keyboard port to find out if a keyboard is present then the next option is to attach a keyboard...
If you can't buy an AT keyboard then perhaps an adapter to convert the current PS2 keyboard to the older AT plug would work.
AT plug -> PS2 keyboard adapters are available for about 3 quid.