What you need is "ipartition". Unfortunately it is $50 US.
The alternatives are somewhat more involved. The manual method that I have used is as follows:
Get WinClone and SuperDuper (both free). Winclone the BootCamp partiton to a file on OSX. Make a SuperDuper backup to an external drive. Go into OSX BootCamp Assistant and restore your drive to a single partition. Reboot. Again go into BootCamp Assitant and create the BootCamp paratition to the new size that is at least as large as the WinClone image. Exit the BootCamp Assistant at the install now or later prompt (select later).
Then run WinClone to restore the image from the OSX file to the new BootCamp partition. A couple of reboots into BootCamp and all is well.
Be advised that this process works, but never do it without making the SuperDuper image off to an external drive (this is called insurance).
Neil.