It has nothing to do with software. I2C is first hardware. Nordic only specifies up to 400Khz. The software is just to make programming that hardware easier on the average user. It is whether the hardware itself is capable of running that fast. Things like internal capacitance, slew rates, current loads, etc, are all concerns.
If you know how to program the nRF51822 registers for the I2C frequency, you can try loading it with 0x10000000 or 0x10040000 and see if you can get it to come close to working. You would need to keep the bus length very short. The pullup resistors may need to be lowered to allow higher current. But you run up against the current limits of the pins themselves are limited to 5mA MAX. And only three pins total can use that amount of current. Perhaps the easiest way to experiment is to redefine one of the variables that the software does define for you. For example change the definition K100, or K250, or K400 from whatever it is to one of the above values. Best have a storage scope or signal ananlyzer available while you experiment.