If you are a Xposed framework user, then you can use the GravityBox module to enable the soft keys. Here are the steps:
That’s all. Reboot your phone and you can see the navigation bar on your handset.
After reboot, Go to Navigation bar tweaks in GravityBox module.
There you can see many options to customize the navigation bar. I suggest you to play with the options and customize soft keys as per your wish.