Fall of Potential ground testing

Can FOP testing be executed with a Megger

Probably not. I am not experienced in FOP testing but:
A megger is designed and used for testing high values of resistance. From my memory of cranked meggers, depending on the model the lowest resistance mark may be 100,000 Ohms or 500,000 Ohms.
Some models would use 6 Volts to measure low values of resistance.
That said, FOP capability may have been added to some newer, electronic meters.
That would then be an instrument capable of both FOP testing and megger testing.
When you say megger to one of us older folk, we assume an instrument that uses a relatively high voltage to measure insulation integrity.
I have seen 250 Volts. 500 Volts, 1000 Volts and 2500 Volts.
Low Ohms testing at 6 Volts is an added feature in addition to conventional megger testing and is not available on most meggers.