you’d basically have to figure out exactly which tables/data that changes each time you add a site. If you have a test server, install multisite, make a copy of the database, then add a site the normal way. Then go through phpmyadmin to view what has changed. Compare to the backup version and you can probably pinpoint exactly what happens. If it’s one table, easy, just import your csv into that one, but it’s probably more than that, but it would probably be faster to import if you have tons of sites. let me know here if you figure it out. I might do the same.