I don't know if it helps, but I have recently put up fencing for a customer. He had a brick wall and purchased 75mm posts for fence panels. The posts had 12mm threaded rods that were secured in the top of the brick using anchor fix resins.
There is no reason why you can't use stainless rods drilled in to the existing posts to extend them. So long as you seal the joints with something like an MS polymer and don't have water ingress, I can't see a problem.