Filling Saline Implants Without Changing Them?

No...once saline implants are filled, the valves closed and the procedure completed, there is no way to "add" saline after the fact. Also, if you plan on reoperating, make sure you discuss the prosthesis warranty and its compromise if the implant is removed to be filled with your surgeon. They are designated by the manufacturer as "One Time Use" items.

Once the saline implants are in, and you have healed from surgery, there is no way to fill them unless you have another surgery.
There is a kind of saline implant which does allow the surgeon to fill it more after surgery. However, these implants have a port which sits on the side of the chest under the skin. It is filled up by injecting the port through the skin. These implants are rarely used.