.. well, actually you can wire it with 6V at the top of the wiper as long as the voltage from the wiper is never allowed to go higher than the MCU supply voltage. If it goes higher the ADC input will fry.
The easiest way to prevent it happening is to use a single resistor from the top of the potentiometer to the 6V supply but as you haven't told us what value the potentiometer is, we can't advise on the value to use. To halve the voltage you probably want one with the same resistance as the potentiometer track. For example if you potentiometer is 10K, add a fixed 10K between the track and the 6V line.
Yes, you can also use an external ADC device that can directly measure 6V but then you have to check you haven't moved the problem to one of logic levels differences between the MCU and ADC.
Brian.