Greetings,
I don't think what you're trying to do is possible. Allow me to explain. The FAT32 filesystem does not have permissions at all. However, Linux requires file permissions in order to function. So, when a Linux system mounts a FAT32 partition it has to mask permission values. This means basically that it takes a given value, and applies it to all files and directories in that partition. This sort of tricks the kernel and allows it to work with the files. Anyhow, to the best of my knowledge, it is not possible to modify the permissions (in this case ownership) of a specific file because the required file permissions don't actually exist, only the mask. Most likely, you will have to move the files to a Linux partition if you want to change ownership.
I hope this helps.
Anyone feel free to correct me if my facts aren't quite right.