You can use the SteamDB Free Packages tool and a browser extension to mass-activate thousands of free-to-play games and DLCs simultaneously.
You cannot add the entire Steam library to a single cart at once. buy all of steam
If you use external keys to add games, Steam typically limits activations to about 50 per hour . You can use the SteamDB Free Packages tool
If you are looking to expand your library significantly, these methods are more efficient: If you are looking to expand your library
Note that Steam’s terms of service clarify you are purchasing a license to play games, not permanent ownership of the software itself. How to Manage Large-Scale Acquisitions
Steam Support generally does not offer a "buy all" package for standard consumers.
Developers often create Steam Bundles that allow you to buy entire franchises or publisher catalogs at a set discount. Mass activate free Steam Games | SteamDB Free Packages tool