How can I add a new partition to my HD without reformatting?

I have a 640GB external HD that has one partition formatted as HFS+. It's attached to a machine running OSX 10.6.6. Now I want to split some of the HD's free space into a new FAT32 partition, without having to reformat the whole HD and losing all my data. I read that I'm supposed to be able to add new partitions in the Hard Disc Utility by clicking the "+" sign, without any loss of data. But in my case the "+" is not clickable and utility says that the partition cannot be altered.

Can I do this without reformatting the whole disc? If so, how?

Select the hard drive (not a partition; the whole hard drive) in Disk Utility and look at the info at the bottom of the window. If it says that your hard drive uses the GUID Partition Table (GPT) then volumes should be resizable. If it says it uses the (Intel PC legacy) Master Boot Record (MBR) format, or the (68k/PowerPC Mac legacy) Apple Partition Map format, then you will not be able to resize the volumes without reformatting/repartitioning the drive to switch it to GPT.