Step-By-Step:

Below are the steps to import a single, standalone site into a Multisite network

1. On the source site go to Duplicator Pro > Packages > Create New: build a package (archive/installer)
2. On the destination Multisite go to Duplicator Pro > Import: Add the archive.zip/daf file from the standalone site
   1. After the archive file is uploaded, click the Launch Installer button
   2. On Step 2 of the import add or copy the recover point (optional) and click the Continue button
      Note: Recovery points are optional but are recommended in the event the install runs into issues.
3. On Import-Installer Step 1 choose Install single site on a Multisite network radio button
   1. Choose the Action drop-down with the action to add as new or replace existing subsite.
   2. Choose the Content Author and set the New Subsite URL slug. This can be any safe URL string.
   3. Click the Validate button and check the results then click Next button and let the installer run.
   4. When the installer is complete Step 2 with show with a status of the install.
      If a recovery point was set then then both an “Admin Login” and “Run Recovery” button will show.
   5. Click the Admin Login to the newly created site and validate the install process.

After everything is done you will have a new subsite installed or replaced in your Multisite based on the action chosen above.