Extracting files from a .zip file will be the same as from a .rar archive using WinRAR, as it handles both types. The actual model isn't of the .rar or .zip extension, they're just archived in that type of file.
If you replace the correct model, it'll display ingame. I'm not sure what the names of the models for The Specialists are called, but they're most likely in the \models\player\ folder. Replace the old model with the one you downloaded, and name it the exact name of the previous model there, otherwise it won't be loaded. It's a good idea to back up all of your previous models, incase you ever want to change back to them.