It sounds to me as if you should look into Type Definitions (aka Typedefs) in LabVIEW, starting with the manual or the online help. When a typedef is passed into and out of a subVI and a new element is added to it, you don't need to make any changes to the subVI connections.
With regard to your question, though: yes, you can modify subVI connector blocks after they are initially created, but it becomes very painful if you have a lot of instances of the subVI, because you have to update a lot of wiring.
Good luck,
John