THIS IS NOT TESTED, AND I THINK IT WILL ONLY WORK IN QEMU!!!!!!!!!!
From a the Fedora 9 main machine (the one you will install VirtualBox) type the following commands:
sudo dd if=/dev/hdb of=qemufedora9.raw
This will create an image called qemufedora9.raw on the path you are, eg. home. But you should have the other's Fedora9 machine hard disk in the main machine to this to work. And that disk may not be hdb!
Then, just call qemu using
qemu -boot c -mem 512 -hda qemufedora9.raw
And QEMU will start with a virtual image of the Fedora 9 machine you want to migrate. You can change the value of -mem from 512 to another value, the value will be the amount of ram used for QEMU. It is recommended that you have kqemu installed. That will bring qemu to an almost Virtualbox-fastness.