Changing hairstyles would be considered a mod. Mods are made by altering the original graphic within a graphic program you have on your computer. So any new hairstyle you want in your game would be created by you. One word of caution though. The game uses files that have a transparent background, meaning there really is no background. If you do not save the transparent background, you will end up with something like this:
Some programs will either not save the transparency or you may need to look for the option to save the transparency. That all depends on what program you are using. Unfortunately, we would not be able to help you with that. You would have to look at the product "Help" files to figure out how to do that.
Good luck!