You can certainly create another VMkernel port on the vSwitch and dedicate it to vMotion. However, without a dedicated physical uplink that won't help much if vMotion kicks in and requires high bandwidth. Anyway, with a separate vMotion port group, you could at least configure the failover order in the port group to e.g. allow vMotion to only use the same uplink as the iSCSI port group in case of an uplink failure (i.e. configure this vmnic as stand-by).
André