Weapons have the fields wAttBonus and wDam Bonus.
(In the editor, select the help menu, go to the "Reference Information" page, and then go to the weapons section to find things like this.)
So, what you want to do in your feat is to search for all Scimitars on the character:
Code:
foreach pick in hero from BaseWep where "IsWeapon.wScimitar"
For each of those fields, you want to subtract the strength modifier, and add the dex modifier, but only if dex is better than strength.
So, let's calculate that difference:
Code:
var bonus as number
bonus = maximum(#attrmod[aDEX] - #attrmod[aSTR], 0)
That will give you your DEX mod - your STR mod, but not let it go negative (so if STR > DEX, it will be 0).
Now, add that modifier to the attack and damage:
Code:
eachpick.field[wAttBonus].value += bonus
eachpick.field[wDamBonus].value += bonus
Here's all that assembled, with the foreach properly closed:
Code:
var bonus as number
bonus = maximum(#attrmod[aDEX] - #attrmod[aSTR], 0)
foreach pick in hero from BaseWep where "IsWeapon.wScimitar"
eachpick.field[wAttBonus].value += bonus
eachpick.field[wDamBonus].value += bonus
nexteach
Now, for the timing - we're working with the attribute modifiers, which means we can't put it earlier than the Post-Attributes phase. The priority within the Post-Attributes phase shouldn't matter, so you can leave that at 100.