It depends on what you're looking to do. If you just want to 'switch' to fat from WBFS, then no, there is no way to do that. However, if you get a partition manager, Partition Magic or something like that, you might be able to resize the WBFS partition, making it smaller, then create a new FAT partition from the new unallocated space. I'm not sure if you can actually do this though, as I'm not sure if the partition managers will know how to deal with WBFS since it's a Wii only thing, but the principle works for other formats, NTFS etc. Be warned though, resizing a partition can take a long time (depending on the size of your drive) so be prepared to be in for a the long run for it.