Short answer, no. You need some device in the middle. A PC/Mac is the simplest. You just plug in the drive to the USB port, and your ipad into USB and use itunes to drag files to your ipad.
Long answer, the device in the middle doesn't have to be a PC/Mac. There are other options like your wifi-router, if it has support for USB.
I've got a d-link dir-505 that acts as a wifi router and storage device. It's made for sharing USB sticks and converting hotel wired connections to wifi, but I've managed to also get a portable spinning HD to work by adding enough power to the USB port (dual usb connector) to spin up the disk. I don't recommend using this device. It's OK, but difficult to configure (you must have a PC to configure it via physical ethernet).
Regardless, the experience of adding storage to your ipad is not the best and you really aren't probably going to like any of the available options.