There are no other official openHAB icon sets as far as I know. There was one published by one community member recently, can’t find it right now. If someone knows, please add a link.
The way you normally add your own icons to your setup is by simply copying them into the icons folder as @Kees_van_Gelder said. See my example here to get an idea: Speedtest-cli Internet Up-/Downlink Measurement Integration