FreeDOS besides QubesOS partition on USB Pendrive

Hello qubes-community,

is it possible to edit the grub.cfg file on the installed qubes.iso on the usb stick to start FreeDOS on other partition? I mounted the main qubes-iso on system with
sudo mount -o loop /dev/sda1 ~/Sandisk/
but it shows that it is only read-only. However when i mount the ANACONDA on sda2 partition it shows that i can read and write.

I thought that i can re-use the Bootloader on the qubes iso on sda1 of my Sandisk USB-Drive. Is it possible to edit the grub bootloader in a way that it also shows up the boot entry for FreeDOS? I installed FreeDOS on sda3 on W95 FAT32 LBA Filesystem with DD.