If I understand correctly, has many through only using existing one-to-many relations.
Currently, there is no way how to graphically create/edit such connections via another existing association, but you can define such a method in the derived class and use existing associations generated by Skipper.
If you have any idea how this could be done in Skipper, please share your idea. But for now, Skipper can edit single/many to many, but I'm not sure how to allow such "many-through" associations.