Then, if it is like you say, risky to use power supply or pb option with standard rc charger, we can switch on battery and use lipo option which can do the job of reducing voltage at the end
NO! It is not just any power source! It is a CC-CV power source, google it. Anyways you need to have some background in electronics to get it. If you dont, don't go that route, just use the original charger.
For example, if you get a normal 26volt 3.8Amp power supply and the battery is empty lets say at 19.8 volts, connecting the power supply to it will cause the power supply to burn out or shutdown.