Disk Utility can do that for you.
If I understand you correctly, you want to resize the single HFS+ partition on the external drive and add a FAT partition, correct?
Open up Disk Utility and select the drive containing the partition you want to resize (usually named something like "320GB <brand name>"). Click the Partition tab, and drag the handle on the lower right of the partition you want to resize up until it's at the desired size, or just click it and choose the size you want. Hit the plus sign under the list to create a new partition, and choose its name, size, and file system. Then, hit Apply to complete the operation.
Note: If done properly this will not destroy your data, but make sure it tells you that! When you hit Apply, it should tell you something like "Partitioning your disk will change one of the volumes. No volumes will be erased." and ask you to confirm it.