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.

icc97

theantichris

This plugin is aimed at developers that need to migrate their databases from 
production to test or local domains.
This is the simplest way I’ve found to migrate databases quickly.
You only have to set the <code>WP_HOME</code> 
(<a href="https://codex.wordpress.org/Editing_wp-config.php#WordPress_address_.28URL.29" rel="nofollow ugc">WordPress address URL</a>) 
and <code>WP_SITEURL</code> 
(<a href="https://codex.wordpress.org/Editing_wp-config.php#Blog_address_.28URL.29" rel="nofollow ugc">Blog address URL</a>) 
constants once for each environment and then the database is automatically 
upgraded when you import a database and login to the admin area.
This stands on the shoulders of the constants and fills the hole for when 
plugins don’t follow the rules and use the database directly.
This plugin is designed to be the most lightweight way to migrate your database 
and stay out the way of your own processes.
It works with WordPress to use WordPress’ own constants to update the database 
using the 
<a href="https://codex.wordpress.org/Class_Reference/wpdb" rel="nofollow ugc">WordPress Database Object</a>. 
It avoids search and replaces in text files. It uses database queries to update 
the database as should be done.
Most developers have their own methods for exporting/importing the database – 
if you can use <code>mysqldump</code> then you probably don’t want a WordPress plugin to 
do it for you.
It makes no assumptions about your database and it’s cross platform.
P.S. Pitta is taken from the start of an <a href="http://wordpress.stackexchange.com/a/182/5433" rel="nofollow ugc">answer from WordPress SE</a>:
<blockquote>
Deployment of a WordPress site from one box to another has been a PITA since 
 day one I started working with WordPress. (Truth-be-told it was a PITA with 
 Drupal for 2 years before I started with WordPress so the problem is 
 certainly not exclusively with WordPress.)
</blockquote>
So this plugin aims to make things less PITA and more yummy Pitta (pedants will 
mention that Pitta can also be spelled pita).
Its inspiration actually comes from the <a href="http://wordpress.stackexchange.com/q/119/5433" rel="nofollow ugc">second answer</a> 
from the same <a href="http://wordpress.stackexchange.com/q/119/5433" rel="nofollow ugc">WordPress SE question</a>.
Coincidentally there is a migratory bird called a <a href="https://en.wikipedia.org/wiki/Pitta" rel="nofollow ugc">Pitta</a>:
<blockquote>
The fairy pitta migrates from Korea, Japan, Taiwan and coastal China to Borneo.
</blockquote>
P.P.S. This plugin is based off the excellent <a href="https://github.com/theantichris/WordPress-Plugin-Boilerplate" rel="nofollow ugc">WordPress Plugin Boilerplate</a> from antichris on Github

Pitta Migration

Migrate WordPress databases using WP_HOME and WP_SITEURL constants.

Export 2 Multisite vs Pitta Migration