How to add NTFS partition on remote server to VMWare ESXi 5.1
Stop thinking about converting the Windows server as that's irrelevant to what you're trying to do.
You can use the local storage of the Windows server as a datastore for your vSphere server, although doing that isn't exactly the way I would go about adding storage to a vSphere server. That being said, here's what you can do:
Download and install the iSCSI Target software for the Windows server. Once installed you can create and configure the software to present iSCSI targets to your vSphere server.
http://www.microsoft.com/en-us/download/details.aspx?id=19867