This example code adds some fixed exchange rates to the Bitcoin and Altcoin Wallets WordPress plugin version 3.4.0 or later.

wallets-fixed-rates.php

<?php
/*
 * Plugin Name: Bitcoin and Altcoin Wallets: Fixed exchange rates extension
 * Description: Example of how to add fixed exchange rates to Bitcoin and Altcoin Wallets.
 * Version: 1.0.0
 * Plugin URI: https://gist.github.com/alex-georgiou/492196184f206002c864225180ca8fbb
 * Author: Alexandros Georgiou <info@dashed-slug.net>
 * Author URI: http://dashed-slug.net
 * License: GPLv2 or later
 *
 */

/**
 * This example code adds some fixed exchange rates to the Bitcoin and Altcoin Wallets WordPress plugin.
 *
 * This script showcases how to provide fixed exchange rates for a coin manually.
 * You can extend it to provide exchange rates from any source.
 * Make sure to include your code before the WordPress init action fires.
 * You could wrap it into a plugin for convenience.
 *
 * @author Alexandros Georgiou <info@dashed-slug.net>
 * @since 3.4.0
 */

/**
 * Adds two new cryptocurrency coins with symbols ABC and XYZ
 * @param array $cryptos
 * @param string $provider
 * @return array
 */
function wallets_rates_cryptos_fixed( $cryptos ) {
	$cryptos[] = 'ABC';
	$cryptos[] = 'XYZ';

	return $cryptos;
}

add_filter( 'wallets_rates_cryptos', 'wallets_rates_cryptos_fixed' );

/**
 * Specifies some exchange rates manually
 * @param array $rates
 * @param string $provider
 * @return array
 */
function wallets_rates_fixed( $rates ) {
	$rates['ABC_XYZ'] = 0.1;
	$rates['XYZ_USD'] = 1.23;

	return $rates;
}

add_filter( 'wallets_rates', 'wallets_rates_fixed' );