Is it possible to reverse a facial feminization surgery that reduced a frontal bone?

Yes, we'd need to look at what was done and what (and why) you want it adjusted.  Custom implants can be made or we can build back the area with a bone cement or other material.  The same incision as before would likely be used though in some situations it could be done with an endoscopic approach.

Any frontal bone that was removed can be reached by either hydroxyapatite bone cement or s custom forehead implant depending on the amount of augmentation that is needed and were the surgical access was from the original surgery.