It's been a while since I was using ReadyAPI VIRTs, but VIRTs support custom properties.
so according to this, it definitely supports properties so I wouldve thought expanding them wouldn't be a problem..... or am I misunderstanding something?
I should have made it more clear that I am asking about the test step REST VirtResponse, not REST Virtual Service.
I wont be much help on this. Ive seen this step type used once about 4 years ago so havent got the experience to comment.
However, the help link you specified does indicate you can create custom properties on the virt response step, so id go with my previous guess that it does in fact support property expansion.