Here are the steps on importing VMWare Fusion disk images to VirtualBox.
First, create a hard link from your VMWare package.
You can move or copy it anywhere you want. What's important is that it's accessible outside the original directory because Mac OS X doesn't treat the package as a navigable directory.
simoncpu@soulfury:~/Documents/Virtual Machines$ ln -h "Debian Linux.vmwarevm/Debian Linux.vmdk"
Next, create a new VirtualBox VM. In the "Create New Virtual Machine" wizard, select "Use existing hard disk" and open the Virtual Media Manager. Click "Add", and navigate to the hard link that you've created earlier.
Click "Finish" to create your VM. In theory (i.e., I haven't tested this) the OS will now boot at this point as long as you've used an IDE controller from VMWare. If not, then proceed as follows:
Change your VM's settings and go to the Storage tab. Next, check the "Enable Additional Controller" checkbox and select any SCSI controller from the drop down box. In the Attachments section, change your hard disk's slot from IDE Primary Master to SCSI Port 0.
Click OK, and start your VM.