There's a 3.5 Feat in Complete Adventurer called Force of Personality that allows this. The feat adds your Charisma modifier to your Will Saves instead of your Wisdom modifier. Have to have a base charisma of 13 to take it. Extremely good feat for Sorcerers and Bards in 3.5.Should be extremely easy to port right over to pathfinder.
Basically a character with Force of personality has a great self image and self worth (possibly a big ego as well lol) so their will power is bolstered by their charisma.
