I'm trying to model steel support at elbow in vertical direction. Using STR modeler to build the element in vertical direction (say from 10 to 20 downward), specifying resting support Y at 20, then include this element into piping file.

Question: how to include friction at support point 20? I don't see any option to specify friction in STR modeler

You cannot specify friction in the Structural Modeler.

When you bring the structure into the piping model, connect the base of that support to a short vertical pipe element. Put the friction on the restraint on the pipe.