For ease of use, constructions in Powerlines Pro constructions are specifications of points in space for conductors to attach to, whether terminating on the pole or tangent to it.  To model a T-junction, one conductor passes through, at a certain attachment height.  The second conductor terminates on the pole - simply place it at the same (or similar) height.  By combining simple constructions in combinations, more complicated constructions can be modelled accurately.