Rather than allocating physical disk space, you'd create a VMFS datastore on the host's disk space and create an additional virtual disk for the VMs on this datastore. Please keep in mind that Windows doesn't support simultaneous access to the same disk (neither physical nor virtual) and trying to do so would cause data corruption. What you should do is to add an additional virtual disk to one of your VM's and share the disk (or a data folder) so that other systems can map it as a network drive.
André