What you can do is to create a logical port on each of the physical switches. Then create a "VRRP/HSRP Cluster" object and add the two logical ports in the VRRP member category. It's a bit redundant because you now have two objects representing the switch stack, but we do not have a 100% representation of VSS/IRF techology at the moment.
I tried it on the demo system but I cannot find, how die create a VRRP/HSRP Cluster.