Storage device probably yes (if you have enough space), storage-target, maybe but I would advice against.
I've moved dozens of VM's from esx to proxmox recently with an iscsi-device as storage, and I found it more safe to set up a second target/lun for proxmox, just to avoid confusion/conflicts, and to more easily clean up once ESX was done.
Also, depending on your exact setup within ProxMox, it might make a lun per disk on your target, instead of ESXi's usual one-big-lun setup (it is possible to use the one-big-lun setup mind you, just a few more steps + commandline, but for me that wasn't an issue, since I wanted to use multipath anyway)
