They all have a "hard stop" on one of the gears, first you'd have to remove that with a hand file or something. Then you'd have to replace the position sensor (usually a pot) with something that could read the additional angle - that won't be so easy to do.
If I were to do it, I would mount the pot external to the servo case, and use some gears to reduce the post travel relative to the servo horn - but that might add a lot of play and jitter.
There are RC "winch servos" for RC sail boats, but I think they are just a continuous rotation servo.
Are you looking to increase a linear deflection, or an increased rotation of what you want to move?