There are two ways to add features:
1) Add them directly on the Plug by downloading and installing packages from the public OpenWRT repositories. This is the quickest way to try out new features.
Please read this article at the OpenWRT wiki to learn how to find out what packages are available and download them.
Be advised that the Smart Plug has just 16 MB of Flash space, so not all packages or combinations you might like will fit.
2) Add them to the Image definition of your build system. The .config file (which can be edited by hand or preferably for beginners, the "make menuconfig" tool). Then you build a new image of the firmware which you then upload in its entirety to the plug. This second method is more repeatable if you wish to flash a number of plugs with the same image.