Fusion 1.1.3 doesn't expose any UI methods to attach an existing drive to a VM, so we'll do it by editing the vmx file with TextEdit. Here are the steps:

1) Download VMware Importer Beta 2 from http://www.vmware.com/download/fusion/importer_tool.html and import your VM to Fusion format.

2) Your imported VM will go into a folder like Documents/Virtual Machines/MyVPCVM. Its disk will probably be Disk1.vmdk. You need the correct and full name for the path, which might be Documents/Virtual Machines.localized/MyVPCVM.vmwarevm/Disk1.vmdk. It's best to get this from a Terminal window, because the Mac will hide package extensions.

3) Use TextEdit to open the vmx file of your existing Fusion VM (the one you're adding the VPC disk to). Add the following two lines at the bottom:

ide1:1.present = "TRUE"

ide1:1.fileName = "/Users/peterwerner/Documents/Virtual Machines.localized/MyVPCVM.vmwarevm/Disk1.vmdk"

being sure to correct my example line with the appropriate full pathname to Disk1.vmdk as it exists on your system.

4) Save the vmx file and quit TextEdit. Start Fusion and start the VM. Your VPC disk will show up as drive E, probably.

(If the path stuff is too complex, you can drag Disk1.vmdk into the original VM's package, and then the line becomes ide1:1.fileName = "Disk1.vmdk" )