Expanding internal µSD storage to ext4?

I'm trying to upgrade my Kobo Glo HDs internal memory, and I would like to use EXT4 instead of FAT32

I have KSM08, and the latest release of KOReader on it.

But going through the filesystem, I have no idea where this is defined, and if I could add or change it without repercussion?
fstab doesn't seem to be used.

I'm sure there's some koboupdate mod that would do this for me but i'm more interested in understanding than ease.