FluidFlow doesn't include or allow an expansion tank to be modeled in-line in a system as this would tend to be a dynamic type of scenario (fluctuating pressure/expansion) whereas in FluidFlow we tend to be solving static system conditions.
Queries in relation to expansion tank tend to be in relation to closed-loop systems and all you need to include in a closed-loop system model is a "Reservoir" node. The "Reservoir" node is available from the Boundaries tab of the Component Palette and this node simply allows you to assign the fluid, its pressure and temperature to the model.
Reservoir Node.