How do I install MS-DOS 6.22 upgrade in a virtual machine without bootable DOS 6.0 installation media?

You can install using 6.22 alone. Boot into the 6.22 install and exit the setup with F3 before you get the upgrade warning and return to the DOS prompt.

You need to FDISK and Format the C: drive. Then you can use the SYS command to transfer the system files to C:. After all this, run the 6.22 install again and you should no longer get the upgrade warning.