Yes, you can use P0.00 and P0.01 as normal GPIOs. But these pins are also used for the external 32.768 kHz crystal, so if you are using the nRF52-DK or using the external crystal in your design, you can’t use them as normal GPIOs without disconnecting the external crystal.
If you are using the nRF52-DK, and don’t have any other available GPIOs and really need P0.00 and P0.01 as GPIOS, you can cut the shorting track on SB1 and SB2, and solder SB3 and SB4 to disconnect the external crystal.
I heavily recommend you to use other GPIO pins if you have the opportunity. Using the external 32 kHz crystal will give you much lower average current consumption than using the RC oscillator.
Anyways, here is a link showing the SB1,SB2,SB3 and SB4.
For using the RC oscillator, you need to use NRF_CLOCK_LF_SRC_RC as a clock source. You normally do this in the function ble_stack_init()
Let's say that you in RX uses 9.2mA, and the LFRC oscillator is configured to 500ppm, and the alterative LF crystal is 20 ppm. Then you have a 480 ppm diff, which gives you an average increase at ~4,5 µA. Adding 2µA for calibrating the LFRC oscillator at 4 seconds interval, gives you in total a 6,5µA increase approximately.
