If the USB keypad shows up in Control Panel -> Game Controllers as a Controller it properly would work in some way. I have no experience with these keypad devices.

A better solution is still a game pad as you would be missing the joysticks on the keypad.
On eBay you can buy a USB game pad at a price below $5 including shipment 
This is one of the reasons why this device type was chosen for the JoyPad plug in.
I have tried one of these cheap game pads and they work.