But grub gives me the error 13 unsupported file (or something simular wording)
Direct chainloading of DOS 6.22 is NOT supported.
It will work for DOS 7.x/8.x, though.
Of course the DOS needs to reside on a FAT12 or FAT16 or FAT32 partition.
The most practical way is since you have triple booting with XP and 7 it is likely you are using NTFS formatted volumem, is to have DOS inside an image.
If you do have a volume formatted as FAT12 or FAT16 and you want to run DOS <=6.22, you need to have the volume have a bootsector invoking the DOS, then:
title 08 - MS-DOS 6.22 INdirect Load io.sys
find --set-root --ignore-floppies /io.sys
Or in the case of a hard disk image (example):
find --set-root /mydos622.img
map /mydos622.img (hd1)
map (hd0) (hd1)
map (hd1) (hd0)
Remember that DOS wants to be booted from first drive (drive 128 or 0x80 in DOS or (hd0) in grub4dos naming )NEVER
use a pre-made entry when experimenting, ALWAYS
use command line, typing command line by line as you will have a much better feedback.