Sorry for the late reply. Didn't read the rest of the thread, so maybe this was already answered by someone else.
I just missed the point that you have to use the SET option at the very bottom of the list (below the C1-3 settings) to achieve the WB K and then store the K value to one of the Custom slots.
Inside the Custom 1-3 you cannot alter the K value, you can only alter the shift (A-B and G-M).