There is no built-in facility to auto-select all products in a given category and add them all to your cart at one time.
You could, with great caution and with your advanced HTML+PHP coding manuals handy, try to construct a special input form that could submit all products via a single click. But its contents wouldn't be built automatically -- you'd have to manually code each link. With even more advanced PHP coding and lots of time on your hands, you could write a script to build such a button for you. As a twist on that, you could build a similarly advanced script to cycle through all the relevant products and do the add-to-cart actions for all of them. You're looking at custom coding to do all that. It can be done though. Perhaps a post to the commercial-help-wanted area might be logical (depending whether you possess PHP coding skills to do it yourself or not)?
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