This app comes with more than 150+ different softkeys to choose from and the growing online database proivides even more user-submitted ones. Watch the video for more info:
Cool, isn't it? Only thing you need is to have navigation buttons (obviously) and root access.
If you don't have navigation buttons, you can always turn them ON - check this post to enable on screen navigation buttons on your Android device.
App is available on Google Play for free HERE. Enjoy!