Hard drive partitioning question

Yes you can format another partition as an another filesystem.

Just click on the drive in Disk Utility, select Partition, then the little [+].

Then pick a size and change the format to MS-DOS (FAT)

 

The best deal is to partition the drive prior to formatting the partitions. There are a couple of solutions:
1) Back up the existing data to another drive and start over.
2) Use a partitioning utility. While I am sure there are many out there, you can check out http://www.versiontracker.com/dyn/moreinfo/macosx/23828

The use of option two will allow you to add another partition without starting over, but you should always back up data before attempting such an exercise, which makes option 1 advisable.