Hi Monique,
by default, external plugins are not supported in Moodle Desktop and Moodle Mobile. However, some of them can be supported in the app if the plugin contains a remote addon (e.g. the certificate module contains it, so it will work in Moodle Desktop). The app doesn't support blocks right now, so the ranking block would be harder to be supported.
Remote addons should be created by the maintainers of the plugin.
You can find more info in here:
https://docs.moodle.org/dev/Moodle_Mobile_Remote_add-ons
Kind regards,
Dani