An addon in a .rar archive doesn't work. The reason why your computer is listing your addons as a WinRAR ZIP archive is because thats what you have set to open a .zip by default. They are still ZIP archives, and not .rar's.
WinRAR was useful before Windows added default file compression. Now it's just a relic of the past.
To make a hat pack with the latest version of hatmod, you need a .zip archive named "Hatmod_YourPackNameHere". Inside that .zip file you place folders named "Hat_YourHatNameHere" containing your hat .dts and any textures it uses. Each hat needs its own folder. Nothing else is needed and you can add as many hats as you like to your zip file. The mod itself does most the heavy lifting.
If you dont know how to create a zip archive, right clicking any file and selecting Send To > Compressed (zipped) Folder will do the trick. You dont need any extra programs.
Honestly, the best way to learn is to copy the structure of existing hat packs.
