Run the file utility over the DMG that you have. If it comes up as having a boot sector in it (aside from a hfsplus filesystem), you should be able to use qemu-img to convert it to something qemu or VirtualBox can use.
Are you speaking from experience? Because that doesn't work since 10.8 I think. You need to use the script to turn the dog into an iso. It is not just a format change, it actually changes the structure of the contents.
