Is it legal to slide my hand back to the pommel on a French grip (posting) to get more reach?

Yes, holding a French grip near the pommel is legal, and so is moving your hand along it during a bout, as long as your hand does not slip backward while you are attacking. The rule is the same for the FIE and USA Fencing: with a plain straight handle you may hold it any way you like and change your hand position during a bout, but the weapon must never become a throwing weapon. Your hand cannot leave the grip, and it cannot slide from front to back along the grip during an offensive action.

So timing is everything. Sliding back while you are out of distance, setting up or waiting is fine. Starting a lunge with your hand near the guard and letting the grip run through your fingers as your arm extends, so the point flies a few extra inches forward, is exactly what the rule forbids, and a referee who sees it can penalize you. Opponents and their coaches sometimes complain about any hand movement at all, so if you post, settle your hand before you go and make your grip changes clearly away from your attacks.

The freedom only applies to straight handles. A pistol or other orthopedic grip must fix one single hand position, with the tip of your extended thumb no more than 2 cm from the inside of the guard, and you must hold it that way. Handles also have length limits: 20 cm at foil and épée and 17 cm at sabre, so you cannot fit an extra-long grip to gain reach. Posting is mostly an épée habit, because the hand and arm are target and every inch of reach helps with stop hits. In foil it is uncommon, and in sabre it is rare.

Is it worth it? Posting buys reach for hits to the wrist and arm, which is why many épée fencers, tall and short, hold the grip near the end. The cost is leverage: a strong beat or bind moves your point further off line, and your parries feel lighter. Try it in drills and a lesson before using it in bouts, and ask your coach whether a long French grip suits your game or whether a pistol grip would serve you better.