1) To define plugin, add below code

<?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="MagentoThemeBlockHtmlTopmenu">
        <plugin name="custom_menu_item" type="MagemonkeysAddCustomeMenuPluginTopmenu" sortOrder="10" disabled="false"/>
    </type>
</config>

in your app/code/Magemonkeys/AddCustomeMenu/etc/frontend/di.xml file.

2) Now, Create Topmenu.php Plugin file at app/code/Magemonkeys/AddCustomeMenu/Plugin/ and paste the below code :

<?php
namespace MagemonkeysAddCustomeMenuPlugin;
use MagentoFrameworkDataTreeNodeFactory;
use MagentoFrameworkUrlInterface;

class Topmenu {
    /**
     * @var NodeFactory
     */
    protected $nodeFactory;
    /**
     * @var UrlInterface
     */
    protected $urlBuilder;
    /**
     * @param NodeFactory  $nodeFactory
     * @param UrlInterface $urlBuilder
     */
    public function __construct(
        NodeFactory $nodeFactory,
        UrlInterface $urlBuilder
    ) {
        $this->nodeFactory = $nodeFactory;
        $this->urlBuilder = $urlBuilder;
    }
    public function beforeGetHtml(
        MagentoThemeBlockHtmlTopmenu $subject,
        $outermostClass = '',
        $childrenWrapClass = '',
        $limit = 0
    ) {
        /**
         * Parent Menu
         */
        $menuNode = $this->nodeFactory->create(
            [
                'data' => $this->getNodeAsArray("CMS Pages", "cms-page"),
                'idField' => 'id',
                'tree' => $subject->getMenu()->getTree(),
            ]
        );
        /**
         * Add Child Menu
         */
        $menuNode->addChild(
            $this->nodeFactory->create(
                [
                    'data' => $this->getNodeAsArray("Abouts us", "abouts-us"),
                    'idField' => 'id',
                    'tree' => $subject->getMenu()->getTree(),
                ]
            )
        );

        $menuNode->addChild(
            $this->nodeFactory->create(
                [
                    'data' => $this->getNodeAsArray("Store Location", "store-location"),
                    'idField' => 'id',
                    'tree' => $subject->getMenu()->getTree(),
                ]
            )
        );

        $subject->getMenu()->addChild($menuNode);
    }
    protected function getNodeAsArray($name, $id) {
        $url = $this->urlBuilder->getUrl($id);
        return [
            'name' => __($name),
            'id' => $id,
            'url' => $url,
            'has_active' => false,
            'is_active' => false,
        ];
    }
}

3) Output :