Replace the WooCommerce registration function

gistfile1.php

/**
 * Custom WooCommerce registration process
 */
function iweb_woocommerce_process_registration() {
	// Duplicate of woocommerce_process_registration() with our modifications
}

/**
 * Replace the WooCommerce registration function
 * 
 * Remove the action during plugins_loaded to ensure that it has definitely
 * been added before we try and remove it.
 * 
 * @author Rob Holmes
 * @link http://goo.gl/UWfbB
 */
function iweb_replace_woocommerce_process_registration() {
	remove_action( 'init', 'woocommerce_process_registration' );
	add_action( 'init', 'iweb_woocommerce_process_registration' );
}
add_action( 'plugins_loaded', 'iweb_replace_woocommerce_process_registration' );