@javier-sandoval share partition is created on first boot of recalbox, post flash, using all the free space on hdd.
So post flash, if you don't boot recalbox directly, and create a fat32/exfat partition on free space (keep 3-5go not used for recalbox. this way it will create the share on ext4 on this space).
once recalbox started, in Option menu you could choose your fat32/exfat partition in usb/external menu.
And after a reboot, OS will use this partition instead of the default one (of 3-5go) in ext4.
I didn't try, but should work.