Você deve usar o dispositivo de loopback para isso:
losetup /dev/loop0 image.img
cryptsetup luksFormat /dev/loop0
Próximos passos possíveis:
cryptsetup luksOpen /dev/loop0 qqq
mkfs.ext3 /dev/mapper/qqq
mount /dev/mapper/qqq /mnt/tmp/
# work with mounted container in /mnt/tmp
umount /mnt/tmp/
cryptsetup luksClose qqq
losetup -d /dev/loop0