In DSS V6 we use iSCSI logical volumes for iSCSI targets and FC logical volumes for FC targets, so it won't be possible to use the same volume for iSCSI and FC target. It's also not possible to convert iSCSI volume to FC volume.
What you can do is to create to separate targets and connect your ESX to both of targets, then move the data from FC target to iSCSI target.