I guess I'm not understanding the question. A SelectionSet at its core is just an ObjectId[]. So when you say add to a SelectionSet you just add the additional ObjectIds to an ObjectId[]. You can collect many SelectionSets combine them all into another ObjectId[] and set the current selection to this array using Editor.SetImpliedSelection. After setting the current selection, SelectPrevious will now pick the objects that were in your ObjectId[].