Unfortunately, there isn't an automatic way to handle ice within MecaStack; however, it is something on our wish list. There are a couple of ways you can try to handle it. The easiest would be to enter it as insulation/refractory with a density of ice and specify that it is external to the stack. This will add the weight and wind area. Typically you only have to check for ice for a reduced wind speed and so you might want to make a separate analysis for the ice case with the appropriate wind speed. Refer to the standard (ASCE 7, etc..) for the wind speed that needs to be considered in conjunction with ice.
In most situation we have found this largely to be an academic exercise because it doesn't usually control the design of a stack.