It is not easy.
The idea is to ensure that all the extensions required by the slave site that you want to import as slave site are correctly installed.
When all the PHP code is present in the master (extensions installed) then you can create a fake slave site where you will re-use the "configuration.php" to replace with your current DB connection.
So the idea is to put the PHP code in the master by the install of the extension
and re-use the existing website to move configuration.php file DB connection to provide the new DB parameter to the slave site.
You will also have to check that you don't have images or specific directory where files or folders must be moved into the new slave site directory structure.
If you want that we help you doing that on your server, we also have billable support that can be ordered at
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