Adding persistence to multiboot USB made with YUMI

I recently made a multiboot USB with YUMI. I've only put the 32 bit and 64 bit versions of Kali on it so I can use the stick on systems that run either a 32 bit or 64 bit OS. I actually have a macbook pro that has a 64 bit processor and an Acer laptop that has a 32 bit processor, and I wanted to run Kali on both of these machines.

I've only run the 32 bit OS on the Acer so far, and everything works great except I can't get the persistence to work. I thought all I had to do was choose the "Live USB Persistence" option, but after some googling I'm under the impression that I need to add a partition to the USB for storage. I don't see a way to do that from YUMI, though.

Is there a way to do this outside of a Linux system? Can I do this without making a live CD of Ubuntu? I really don't want to make one JUST so I can use GParted. Is there something else I need to do because this is a multiboot USB, or because I used YUMI to make it? I'm open to downloading an application on Windows Vista (it's an older Acer) or on OSX.

Am I completely wrong about what I think I have to do to make Kali persistent? Am I overlooking something? Any advice would be appreciated.

EDIT
Also, once persistence is working, I'd like to use a single space to store things for both the 32 and 64 bit versions of Kali, preferably in a space larger than 4GB. It's on a FAT32 file system so I know about the 4GB limit. I've read that it's possible with casper -rw partitions, but I don't know it's feasible because of the ways the operating systems are processed. If I need to make 2 separate spaces it's ok, just less convenient if I want to add the same documents or applications to both Kalis.