If your WBFS drive is disk0 (first partition,) then what you're implying wont work, more so, you don't want to. I'll avoid the technical details, but just give you the best option.
Dump your FAT drive to your computer or another drive. Then go from there.. you should truly try to make your WBFS partition disk0, as it avoids a lot of complications with getting things to read it.
If you can't dump your FAT drive, then things get messy. As mentioned, you need to do a d2d transfer. Set your partitions like..
WBFS(300g) (disk0)
FAT(50g) (disk1)
WBFS(150g) (disk2)
150g "should" cover your 40 Wii games. From here, you can reformat disk0 into..
WBFS(200g) (disk0) *new*
FAT(100g) (disk1) *new*
FAT(50g) (disk2)
WBFS(150g) (disk3)
Do another d2d transfer from disk3 to disk0. From here, you'll want to transfer your FAT data (movies or whatever) from disk2 to disk1. Now you can break disk2/disk3 and combine them to disk1 to have your 200g/300g. Adding partitions to the front and end of existing partitions is something you really want to avoid (thus our fat to fat transfer.)
Just remember to try and get your WBFS drive as disk0 if you can. -ciao