You can migrate sites that use their own domain names, as well as sites that use a subdomain on the primary domain.
Steps to follow
1. Backup your site
Generate a full site backup. It might also help to copy all the files so that you can easily access the files for plugins and themes, which you’ll need in a later step.
2. Export from your existing WordPress
In each of the existing WordPress installations, go Tools > Export in WordPress.
Exporting WordPress data (posts, pages, custom post types, comments, custom fields, categories, tags, custom taxonomies, and users) is sometimes necessary and useful.
When you click the Download Export File button, WordPress will create an XML file for you to save to the computer.
Make sure that the export file actually has all the posts and pages. You can verify this by looking at the last entry of the exported file using the text editor. The last entry to be the most recent post.
Note: widget configuration and plugin settings are NOT exported in this method.
3. Export widgets from existing WordPress
Insert Widget Importer & Exporter plugin to export widgets.
Click the Export widgets button. Widget Importer & Exporter will create an export file (in JSON format with .wie extension) from currently active widgets.
4. Export customizer settings from existing WordPress
Insert Customizer Export/Import plugin to export widgets.
Click the export button from the customizer and a file will automatically begin downloading with your settings. The export file is named after the theme and can only be used to import settings for the theme or child theme that they came from.
5. Export plugins from existing WordPress in zip format
6. Export theme from existing WordPress in zip format
7. Create a new site in the Multisite
8. Install and activate the plugins
9. Install and activate the theme
10. Import WordPress file
Import the exported file.
Browse the file and click the Upload file and import button.
11. Import widgets from the exported file
12. Import customizer settings from the exported file
After successfully import all things new WordPress site is ready for the public.