I was able to get my Blue Gecko to perform a conversion. I solved it by learning that I needed to select the ADC_CLK input by writing directly to the CMU_ADCCTRL register to specify the AUXHFRCO.
Is there a example of programming the VREFATT and VINATT? This example would help in verifying a correct set up of the ADC reference and input pin.
Your question is not on the ADC so it's unrelated with the Bluetooth stack. I moved it to the 32-bit MCU board where you can get support on the peripherals.