Hello,
Thank you for posting in Q&A forum.
I understand your requirement, but we can’t directly sync NAS to SharePoint online, you would need to copy/upload the files manually. Sorry for the inconvenience.
You can use OneDrive sync client to sync your SharePoint library to your local computer and then use File Explorer to move/copy files from your NAS drive to SharePoint.
Please refer to:
Sync files with OneDrive in Windows (microsoft.com)
Sync SharePoint and Teams files with your computer (microsoft.com)
Best regards,
Jill Zhou
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