Looks like everyone missed the specific question. You have an existing BC partition. Fusion can use that for virtualizing a Windows environment without requiring a reinstallation of Windows, but still relies on the existence of Windows in the BC partition.
Virtual machines do not require separate partitions, but rather a minimized portion of storage commonly called a "bottle" to store the Guest OS's environment.
If you were to create a new VM for Windows without relying on the BC partition, a bottle would be created for it and allow you to install Windows. Whether there is a way to transfer the existing environment into that bottle, I don't know. You could try Ghost, or something similar, but I don't know of an easy, seamless way to accomplish what you're looking to do.
If you install Fusion and point it to the existing BC partition, then delete the BC partition, Fusion will require a reinstallation of Windows (actually, a completely new VM), as it no longer has the original environment to refer to.