jamiechong

Export 2 Multisite exports your standalone database SQL for import into a WP multisite installation. It handles duplicate users (based on email) and will copy over all site settings including Widgets and Menus. You still need to copy the files for your themes, plugins and uploads.
<ul>
<li>This plugin has been minimally tested, but has been used to successfully migrate a handful of standalone sites to a multisite installation.</li>
<li>There is a good possibility it won’t work for sites that make use of a multitude of plugins – especially plugins that have their own database tables.</li>
</ul>
Installation
<ol>
<li>Create a new site on the Multisite.</li>
<li>Copy the plugins and themes from this standalone site to the Multisite.</li>
<li>Copy the files from wp-content/uploads/ on this site to wp-content/uploads/sites// on the multisite.</li>
<li>Backup the database on the Multisite.</li>
<li>Provide the Site ID of the Multisite. This can be found by hovering the mouse over the new site on Network Admin->Sites.</li>
<li>Provide the New URL. Example: //mysite.my-wp-multisite.com</li>
<li>Provide the New file path (The file path of the Multisite installation).</li>
<li>Export the database dump.</li>
<li>Import the SQL into your Multisite installation.</li>
</ol>
<ul>
<li>Important: The exported SQL will DROP the tables that were created for the Site ID you provided. So only perform this migration on new empty Sites.</li>
<li>Note that the you may need to review the exported SQL to ensure that plugin database tables have been exported correctly.</li>
<li>The site admin (username and password) is the user you created while creating the new Site, NOT the site admin of this standalone site.</li>
<li>The guts of this export plugin were inspired by <a href="https://iandunn.name/comprehensive-wordpress-multisite-migrations/" rel="nofollow ugc">this blog post</a></li>
<li>The code for this plugin has been modified from the <a href="https://wordpress.org/plugins/wp-migrate-db" rel="ugc">WP Migrate DB</a> plugin, but should not conflict with it in any way.</li>
</ul>

Export 2 Multisite

Exports your standalone database SQL for import into a WP Multisite installation.

Export 2 Multisite vs WP Import – Ultimate CSV XML Importer for WordPress