You can already do this:
Settings----> Downloads + Cloud ----> Steam Library Folders.
Here you can add you Windows Steam Library.
OR
Create a symbolic link pointing from "/home/'user acccount name'/.local/share/Steam/SteamApps/" to your (windows) steamapps folder.
As I understood, he already does share the files..
I think some more people requested this already
A workaround atm is to use the steam wine version
Nothing extracted yet.
It would be a great feature - as I dualboot for some games, to be able to also download Windows games whilst in the actual Linux client (both use the same linked to SteamApps folder).
As I do most of my work in Linux and only sometimes boot up Windows for some games, this would be a great feature as it wouldn't require me to be in Windows to grab new games.