It looks like it's a little tricky to get a custom roll into the crit section on that sheet, due to the way it calculates the field. The easiest way would be to create a button in the description section (or the notes section, but the description is slightly cleaner looking IMO). First, create an Ability macro on the Attributes & Abilities tab of the character sheet, called corrosiveBurst, and save this as the content: &{template:pc} {{name=Corrosive Burst}} {{type=damage}} {{damage=1}} {{dmg1=[[3d10]]}} {{dmg1type=Acid}} {{dmg1flag=1}} Then, tick the Description check box for the attack and throw this in the input box: [Corrosive Burst](~bob|corrosiveBurst" style="color:green) damage on critical hit. replacing "bob" with your character name. You can just use [Corrosive Burst](~bob|corrosiveBurst) to create the button link, but the PF sheet makes it black like the rest of the text, the colour just helps it stand out a little. You'll need to manually click on the link when you roll a crit, but it's much easier than the alternative. To get the damage to roll automatically on a crit, you'll need to reconstruct the entire crit calculation and find some way to force it into the sheet macro, probably via a custom condition Attribute since those are called at the end, allowing you to overwrite the crit field with your own code. I don't know the sheet well enough to see exactly how to do it.
