Well, maybe it's just me, but I think trying to edit a partition in Windows, regardless of the tool being used, is a massive pain. I've never once run into an issue with gparted, and you can boot it from USB. Naturally it does double-duty (it can edit your computer's internal HDD, in case of a problem there) so it's great to have around by itself or in a linux distro.
Either way, without being able to see the current configuration of your disk, its hard to give advice, but here goes.
So, you started with a single partition, as below
[---------FAT32--------]
And what you want instead is
[--FAT32--][--WBFS--]
Correct?
So, where you're trying to create a new partition and it won't let you.. is it an "unused partition" or is it "unallocated space"? If its an unused partition, then you're basically done- that just means it isn't formatted, and if you format it to wbfs, you're set. If its unallocated space... Well, then it seems you may have a deeper problem.
Regardless of the above, psyblade is right. Since you have to use a WBFS manager to put games on the partition anyway, it is much simpler to use an FAT32 partition and convert your games to .wbfs format. You can use Wii Backup Manager to do so, it has a fantastic UI, and it can also convert .wbfs back into .iso, should you ever need it to for whatever reason.