How to get started with interfacing USB devices to iPad?
You cannot directly interface with generic USB devices from a stock OS iPad unless you are a manufacturer in Apple's MFi (http://developer.apple.com/programs/mfi/) program.  There appears to be a ton of paperwork involved to just apply.  The external accessories API is of no help on stock OS devices, unless you are manufacturing your own custom MFi-compliant device.

Alternatives


If your USB device can pretend to be a MIDI instrument or controller, you might be able to interface with it over USB by sending/receiving MIDI messages from the iPad.
Another option is to use a small inexpensive board, such as a Raspberry Pi Zero W, and use it to programmatically transfer USB data via WiFi to an iOS device.