Before you install anything or run out and buy anything, did you at least try and plug the external drive into the iMac? OS X has been able to
read Windows formatted drives (NTFS) since Panther (10.3 if the following link is right. That would be almost 10 years of NTFS support).
http://macintoshgarden.org/forum/ntfs-supportOS X cannot
write to NTFS (out of the box at least). If you're importing from Windows format to Mac format, you shouldn't be writing to NTFS so you should be ok.