The workaround mentioned above is what worked for me:
- Add a port group to a regular vSwitch: esxcfg-vswitch -A "iSCSI3" vSwitch1
- add the new vmkernel nic to this portgroup: esxcfg-vmknic -a -i DHCP -p "iSCSI3" -m 9000
- use the GUI to migrate the new vmk to the desired dvSwitch and adjust the IP configuration
To confirm the membership of the vmk, just run: esxcfg-vswitch -l
To check the mtu of the newly created vmk: esxcfg-vmknic -l
That's all.