- Download the apk of the app that you need from the internet (you can do that directly via the browser or using your PC and a USB drive or memory card, ...), there is a lot of websites to get that or you can copy it from any rooted android device.
2 - After installing kodi, run it, go to "Programs" >> "Android apps" (here you will get all the installed apps in your box) >> "Settings" (here you will get the settings window of android), then "Security and restrictions" then turn "unknown sources" to "on", and "verify apps" and "restricted profile" to "off".
3 - Then back to kodi >> "Programs" >> "Android apps" but this time go to "AppInstaller" (the app which we will use to install apks), then select your source (local disk, memory card, usb drive), the app will search all available apks, then you have just to pick (check it at the right, don't click) the one that you want to install and on the menu at the top right, click "install
