You should make a family for this, start with a window family, add your plaster trim as an extrusion or sweep, set up any parameters you need( window families will allready have height and width so you'll want trim thickness and trim width or something like that) then load into your model.
Note if you don't want it to schedule as a window you can change the family type to a generic model or structural framing family under Settings>Family Category and parameters in the family editor.