Partitioning my external HD questions

ok, i have a 500 GB external that ive been using since december and it is great. the only problem i have is that it is formatted in FAT32. the obvious reasoning for this is because macs can not write to NTFS, they can only read. the downfall of FAT32 is that you can not have files over 4GB. it just cant do it. i recently found a way to get the amc to read AND write to NTFS! (yay!) <see here>. however, i have about 300GB + worth of stuff on my external and i want to convert it to NTFS but i dont know how to partition it without losing everything else. i have iPartition but it wont let me access my external (weird...) so does anyone know a way i can partition the free space to NTFS without losing my existing partition?? thank you in advance!

If you have a Windows machine, you can use "Partition Magic" to resize the partition.

that was one option i had already thought about but i just wanted to see if there was a way to do it in osx is all. thank you though.
-Philip