ESXi won't automatically format your storage, except maybe at install. If you can take the volume offline during install or remove it completely during install, then you should be safe. Just re-attach it after install and boot up the host. It should find the storage during boot-up scans and add it to your datastores. If it's not there then it might be seen as a snapshot lun, you will know that when you start the create datastore wizard. Don't complete the wizard if it doesn't see it already as you might reformat your datastore.