How to extend bootcamp partition?

Solution 1:

Late note: iPartition cannot handle CoreStorage or APFS volumes & is no longer supported by its author. Coriolis is now a single page site, with unsupported free license downloads for some of its old software.
If you are still on a pre-CoreStorage macOS, then it is still very much the tool to use.

For all my partitioning, I use and recommend iPartition.

iPartition allows you to modify Boot Camp partitions just like regular partitions. iPartition will reconfigure the Windows partitions automatically.

You can use iPartition on drives that contain Boot Camp volumes; in fact, you can even resize your Windows partitions, and let iPartition take care of reconfiguring the Windows boot process for you.