The implant manufactures have a recommended range for filling of saline implants. Assuming your implants are not inflated to maximum recommended volume, it should be a safe and relatively easy procedure to add 25cc of saline. I, however, would not recommend this procedure. A 25cc increase in volume is small and would not provide substantial added augmentation. It therefore is hard to justify the costs and potential risks for even this relatively minor procedure.
I hope you are not dismayed by this answer. Best wishes to you.