The storage network consist of the vmkernel port so your not going to be able to utilize that to connect the VM. You would need to define a new portgroup under the Virtual Machine network that can connect to iSCSI and add a second NIC to the VM utilizing that port group. I have to agree with athlon_crazy in that it may be easier just to use VMFS or RDM to add secondary storage to the guest VM unless you have a specific reason not to do that.
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