Add a declaration of something involved at the beginning of the function. It does not need to be used, as long as the con/destructor is being called. An int was not enogh, but e.g. an QStringList did the trick. Split the string literal in one of the blue lines to something like QString("k") + "Hz". Change switch(index) to switch(index+1), with obvious changes to the case labels.
if (index == FREQUENCY) { // Add the Hz items } else { // Add the Seconds items }