This is on my wish list and I'll add a vote for you. Meanwhile, you can put whatever text you want to into the Key field. I sometimes put "A (D)" to show the actual key plus the harmonica key for the harmonica player. Similarly, you could use abbreviations like "A (S=Bb, J=C)" to show that a song is normally in A, but Susie sings it in Bb and Julie sings it in C.
Another option is to create custom fields like "Susie key" and "Julie key" and populate those with "Bb" and "C" and then when you print or email a set list, you can hide the Key field and show the appropriate custom field instead. That's assuming you have one of those vocalist for the whole show, rather than different vocalists on different songs.