How can I upload a directory structure from a PC to iCloud without first copying it to my iCloud drive? I have a directory structure of about 12GB on a network drive that I'd like to copy to my iCloud storage. I don't want to copy the 12GB from the network drive to my local iCloud drive folder just so it can be copied from there to iCloud. 

Other cloud storage services (e.g., Google Drive, Microsoft OneDrive, Dropbox) support a browser upload interface that makes it possible to drag folders to be copied into cloud storage onto the browser, but I can't find an equivalent kind of functionality for iCloud. Is there some way for me to directly copy my data from its current location on a network drive into iCloud?

I'm running 64-bit Windows 7 Professional.
As of macOS Sierra 10.12.5 (and the equivalent iCloud for Windows), iCloud Drive does not offer a facility for selective-sync of files between your local storage and the cloud.

Therefore, anything you upload to iCloud Drive using the web interface would then make its way back down to your local storage on any machine logged in to iCloud.

iCloud Drive doesn’t appear to be the archival storage solution you’re looking for. It’s a full-syncing service.