Magento 2.1 – Add New Method to Block Class

I need to add a new function to the configurable.php block, located in Magento\ConfigurableProduct\Block\Product\View\Type\Configurable. It's a function similar to $block->getJsonConfig() except it gets some different values.

Can I do this (add a new method/function) through a plugin or do I have to override the original block? I'm not changing or removing any of the original functionality, just need to be able to call another function from the phtml file. If I override, does that mean I need to copy all the original methods from the original block php or can I just define the new function I need in my override file?

Edit: I have followed Aaron's advice for extending the original class. Here is my di.xml file:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference
        for="Magento\ConfigurableProduct\Block\Product\View\Type\Configurable"
        type="zzTest\zzModule\Block\Product\View\Type\Configurable" />
</config>

Here is what zzTest\zzModule\Block\Product\View\Type\Configurable.php looks like:

<?php 

namespace zzTest\zzModule\Block\Product\View\Type;
use Magento\ConfigurableProduct\Block\Product\View\Type\Configurable;
class Configurable extends \Magento\ConfigurableProduct\Block\Product\View\Type\Configurable
{
    public function getXPPackCounts()
    {
        return 'good';
    }
}

Finally, I attempt to call this function from configurable.phtml in design/frontend/my/theme/Magento_ConfigurableProduct/templates/product/view/type/options/configurable.phtml . My calling code looks like this:

<?php /* @escapeNotVerified */ echo $block->getXPPackCounts() ?>

I know this phtml file is being called okay because I have other stuff on it that shows up. But I don't receive any output from my call. What have I setup incorrectly?