Go on your main account and play a game (you can even start a custom game and leave when it starts).
Then go to the folder "C:\Riot Games\League of Legends\Config" and set the files "game.cfg" and "PersistedSettings.json" to read-only.
Now start a game on every account you want to copy the settings on (even custom game is ok again).
Once you copied the setting on every account you can uncheck read-only to the files if you want to be able to change the settings again.