There are programs like Partition Magic (just one example) which do non-destructive partitioning, however it's important to note that they can and do sometimes screw up, causing you to lose ALL data on the drive.
So, make sure you have a backup of anything you wouldn't want to lose. The odds of something bad happening may be small, but that will be little consolation if you end up being one of those with the bad luck to be affected.