Magento – Magento 2 how add extension attribute to order item

Create a extension attribute for the required datainterface here I'm providing sample:

<extension_attributes for="Magento\Store\Api\Data\StoreConfigInterface">
    <attribute code="sample" type="string"/>
</extension_attributes>

Have a plugin on the interface di.xml

<type name="Magento\Store\Api\StoreConfigManagerInterface">
    <plugin name="nameofplugin"
            type="Namespace\Modulename\Plugin\filename"/>
</type>

Plugin class

    <?php

      namespace Emc\App\Plugin;

      use Magento\Store\Api\Data\StoreConfigExtensionInterface;
      use Magento\Store\Api\Data\StoreConfigInterface;
      use Magento\Store\Api\Data\StoreConfigExtensionFactory;
      use Magento\Setup\Exception;
      use Magento\Directory\Model\Currency;
      use Magento\Store\Model\StoreManagerInterface;


/**
 * Class classname
 */
class classname
{
    protected $storeConfigExtensionFactory;
    protected $storeConfigInterface;
    protected $storeConfigExtensionInterface;
    protected $scopeConfig;

    /**
     * Product constructor.
     * @param \Magento\Catalog\Model\ProductRepository $productRepository
     */
    // objects need to be changed based on your requirement 
    public function __construct(
    Currency $currencyFactory, 
    StoreManagerInterface $storeManager, 
    StoreConfigExtensionFactory $storeConfigExtensionFactory, 
    StoreConfigInterface $storeConfigInterface, 
    \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
    ) {
            $this->storeConfigExtensionFactory = $storeConfigExtensionFactory;
            $this->storeConfigInterface = $storeConfigInterface;
            $this->scopeConfig = $scopeConfig;
            $this->currencyFactory = $currencyFactory;
            $this->storeManager = $storeManager;

     }


    /**
     * @param string[] $storeCodes list of stores by store codes, will return all if storeCodes is not set
     * @return \Magento\Store\Api\Data\StoreConfigInterface[]
     */
    public function aroundGetStoreConfigs(\Magento\Store\Model\Service\StoreConfigManager $storeConfigManager, \Closure $proceed, array $storeCodes = null)
    {

        $result = $proceed($storeCodes);
        foreach ($result as $storevalues) {
            $storeconfigExtension = $this->storeConfigExtensionFactory->create();
->getCurrencySymbol());
            $storeconfigExtension->setSample($storedKey); //attribute code

            $storevalues->setExtensionAttributes($storeconfigExtension);
        }
        return $result;
    }

}

Not sure, but the first thing I noticed in your code is that you write plugin for \Magento\Quote\Api\CartRepositoryInterface but should be for \Magento\Quote\Api\GuestCartTotalRepositoryInterface

Upd #1: The following works for me:

extension_attributes.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="Magento\Quote\Api\Data\TotalsItemInterface">
        <attribute code="some_super_cool_attribute" type="string" />
    </extension_attributes>
</config>

module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Underser_CartExtensionAttributes" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Quote"/>
        </sequence>
    </module>
</config>

di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Quote\Api\GuestCartTotalRepositoryInterface">
        <plugin name="underser_add_extension_attributes" type="Underser\CartExtensionAttributes\Plugin\Quote\TotalsPlugin"/>
    </type>
</config>

\Underser\CartExtensionAttributes\Plugin\Quote\TotalsPlugin

<?php

namespace Underser\CartExtensionAttributes\Plugin\Quote;

use Magento\Quote\Api\Data\TotalsInterface;
use Magento\Quote\Api\GuestCartTotalRepositoryInterface;
use Magento\Quote\Api\Data\TotalsItemInterface;
use Magento\Quote\Api\Data\TotalsItemExtensionInterfaceFactory;

class TotalsPlugin
{
    /**
     * @var TotalsItemExtensionInterfaceFactory
     */
    protected $totalsItemExtensionInterfaceFactory;

    /**
     * TotalsPlugin constructor.
     *
     * @param TotalsItemExtensionInterfaceFactory $totalsItemExtensionInterfaceFactory
     */
    public function __construct(TotalsItemExtensionInterfaceFactory $totalsItemExtensionInterfaceFactory)
    {
        $this->totalsItemExtensionInterfaceFactory = $totalsItemExtensionInterfaceFactory;
    }

    /**
     * After get items.
     *
     * @param GuestCartTotalRepositoryInterface $subject
     * @param TotalsInterface $result
     * @return TotalsInterface
     */
    public function afterGet(
        GuestCartTotalRepositoryInterface $subject,
        TotalsInterface $result
    ) {
        /** @var TotalsItemInterface $item */
        foreach ($result->getItems() as $item) {
            $extensionAttributes = $item->getExtensionAttributes();

            if (!$extensionAttributes) {
                $extensionAttributes = $this->totalsItemExtensionInterfaceFactory->create();
            }

            $extensionAttributes->setSomeSuperCoolAttribute('someSuperCoolAttributeValue');

            $item->setExtensionAttributes($extensionAttributes);
        }

        return $result;
    }
}