Generally speaking, you cannot usually convert random videos to APK format. Nevertheless, some apps can use images or even videos to create rich and sometimes animated wallpapers for Androids. These are generated as APKs, i.e. programs that are installed and run in the device to replace the wallpaper.
So mpg to apk conversion it is really not a file conversion per se, more of generating a simple Android app that contains the image your video you import in to.