Do you have another server for the ESXi host or are you wanting to reprovision the existing hardware as your ESXi host? If you are planning to reprovision the hard ware it is going to be difficult since ESXi installs on to the bare metal and will overwrite the exisiting o/s - if this not what you are [;anning but to instead stand up a new host than your can use vmware converter to P2V the Windows 2008 server to a virtual machine.
Now on to you underlying question - how can you share data between VMs is you would have to treat like you would physical machines through the use of CIFS or NFS shares which can be hosted by virtual machines.