Hello there,
The key point is that you are going to import updates files from local drive .
Currently , in WSUS , we can only manually import updates directly from windows update catalog , not local drive .
As a workaround , I'd suggest you create a "online" WSUS server and then "replica" it to that WSUS server which can not connect to Internet .
Similar discussion here https://social.technet.microsoft.com/Forums/Lync/en-US/86105f02-de0f-4230-a1ad-a865b8a08439/how-import-manualy-updates-in-wsus-from-local-hdd?forum=winserverwsus
Hope this resolves your Query !!
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