1) Model the outer stack diameter and thickness using the "Stack Geometry" screen.
2) Calculate the weight of the inner shell in therms of weight per length (lb/ft or kg/m). Then enter this as a general attachment uniform, and consider the area to be zero.
This method will account for the mass distribution for the inner shell, but it will not take any consideration of the strength added by the inner riser. Although this is not 100% accurate, generally the inner shell does not contribute a great deal of strength to the system since it is smaller in diameter and thickness.