It is possible to do this but also dependent on the initial surgical approach. However, it probably voids the warranty and, if you developed a complication, which is possible whenever the cavity is opened, you would be responsible for its replacement. If you had an axillary approach, this could definitely not be done.
Generally, the cost of a new saline implant isn't large compared to the cost of going back in, so I generally destroy the old implants while removing them, and put new ones which are the size the patient wants. Each saline implant has a range of size. I usually fill an implant to it's middle or high end of the range, so there is usually not much that can be added anyway.