Yes, Apple makes a Thunderbolt Gigabit adapter. I'm using it on my Mac Mini 6,2, and it is directly connected to NIC 2 on my NAS with a CAT 6 cable.
It's not the ideal solution. I'm not going to get into the details here, but I cannot have more than one iSCSI target on my NAS because if my ESXi host is reset, only the primary target can be automatically re-connected to the ESXi host. As long as you are OK putting all of your VMs on the same target, you should be fine, though.
The better solution is to use a Gigabit switch, but the connection between the ESXi host's NIC2 and your iSCSI NAS' NIC2 should either be on a separate VLAN from the rest of your network, or if your switch cannot create different VLANs, this connection should be on a separate Gigabit switch.
In ESXi, you should limit iSCSI to only NIC2, and use your primary NIC for administration of the ESXi host and for the LAN connectivity of your VMs hosted on the ESXi host (your Mac Mini).