Can a water softener be installed after a thermal expansion tank?
I don't think this should be a problem, water softeners I have seen do not have check valves in them so water can flow through both ways. It's only after a check valve where expansion capacity is needed.
Many homes do not even have an expansion tank, they are most frequently found (at least in my area) where there is a pressure regulator (which contain a check valve) installed after the meter. In that case you need an expansion tank (especially with a water heater) because of the additional pressure warmer water will create in the system which needs to be relieved. If there are no check valves in the system the entire city main can serve as the expansion tank (water flows backwards through your meter).