How to use autoprefixer for external css files - webpack

I have all the css files inside ${ROOT}/resource/css/. While building using webpack, I want to copy all the css files to ${ROOT}/build/css/ with desired vendor prefixes added to all the css rules. Currently I am using copywebpackplugin to copy all the css files as is to ${ROOT}/build/css/. I am struggling to add add a step to add vender prefixes. Please help.