You can NOT size the NECK completely without also sizeing the body of the case with a fl. size die. In fact, most times you will size quite a bit of the body if you size much of the neck at all!
What you CAN do with a fl. size die is,to set it toNOT push the shoulder "back" and only "partial size" the body of the case by ajusting the fl. die so it "doesn't" size the full length of the neck.
Another problem too with using a FL sizer to neck size, is if the case is squeazed at all below the neck by the sizing die without being set to bump the shoulder back at least slightly, you could end up lengthening your case enough that it won't chamber, not to mention the possibility of moving the neck out of alignment with the case body.