Here you have some info found on other german forums, I have not tested it yet myself but it should work, to make it permanent you will need to copy is to OpenXCAS with some script that copies files and executes command. The tmp directory will be erased on reboot.

Copy the .ko file via FTP to /tmp
Then connect with telnet to the box and enter the following command:
insmod /tmp/usbhid.ko

If no errors occur, try to connect a USB keyboard to the box.