Resizing the BootCamp Partition

Hi all,

I am trying to resize the boot camp partition on my iMac. I opted for the 32gb option when I first ran boot camp but I wanted to add more. I read online I could do this my shrinking the Mac HDD and then creating a new partition FAT32. Then open up windows and open up disk management. Then right click my bootcamp hdd and click "extend volume." however, that option is greyed out. am i missing something or is there some other technique? thanks for the help!