Well, the feat only applies vs mind affecting spells and abilities, I don't know you should fiddle with the base Will save.
If you want to anyway, I'd say make it an activation, and give it this code, around Post-Attr 10000:
doneif (field[hIsOn1].value = 0)
hero.child[vWill].field[Bonus].value -= hero.child[aWIS].field[aModBonus].value
hero.child[vWill].field[Bonus].value += hero.child[aCHA].field[aModBonus].value
As always, no assurances on the code, make sure you test and refine if necessary.
No need to refine. Works as is.
Two things. First, to make it an activation, select "Show in Charges List?" and put something in "Charge Effect 1" such as "active" or anything that will let you know what's going on there.
The second is more important. I recommend creating your own .user file (call it anything you want, but I would avoid following the naming convention of the community set for personal files), then use the New (Copy) option to make a copy of the Force of Personality feat. In the Replaces Thing Id box, put the id of the original feat (fForcePers). The reason I suggest this is that if you change the feat in the community file (in this case CAdv - Feats), it will be overwritten with each update and you'll have to do this again. If you create your own file, you won't have to worry about it.