The question is: How can I assign an IP address to the USB printer in the Aiport Extreme ?
IP addresses can only be assigned to network clients that connect to a router using either wired Ethernet connections or wireless.
The USB port on the AirPort Extreme will only support a compatible printer. It does not support scanning, copying, maintenance, or other advanced features.
If you need one of these features using USB, you will need to attach the device directly to your computer.