This task is easy to accomplish. Go to the Proxy Variables panel (the second tab after the Property panel) and expand a hub proxy variable that you want to update. There will be a drop-down list of objects linked to that proxy variable. Then drag and drop a new object, e.g. element of the same type, to the list. You can also delete objects in this list and rearrange their order (the latter can be relevant to the proxy array variables). The only limitation is that a single proxy variable cannot be turned to hub proxy, and vice versa, by manipulating with the list. The hub and array proxy variable have to be created from the start.