If VMware Fusion does not recognize it as a Boot Camp partition (which it did in earlier versions) then you'll have to use vmware-rawdiskCreator to create a meta-data .vmdk virtual hard disk to use the external drive as a Raw Disk in a Virtual Machine.
In a Terminal copy and paste the following command and then press Enter. This will output the internal help file showing command syntax.
"/Applications/VMware Fusion.app/Contents/Library/vmware-rawdiskCreator"
The above pathname for vmware-rawdiskCreator is for VMware Fusion 4.x. Previous versions of VMware Fusion store vmware-rawdiskCreator in a different location, being "/Library/Application Support/VMware Fusion/vmware-rawdiskCreator".