Apply difference between an Attack roll and AC to Damage

Can I do this.  If so how?  As an example - If the to Hit number is 8, and I roll a 14, then I would add 6 to my damage roll.

This probably belongs over in the Macros forum. Which game/sheet are you running?

I am building a sheet, so I'd prefer not to do API.  This is a sheet for Mongoose Traveller 2e.  When you make a roll, the difference between that roll and the target number is called effect.  It can be positive or negative.  In combat, a positive number gets applied to damage. 

You cant do this if you want to show the attack and damage separately. If you just want to show the final result, and dont need to show hit and damage separately it can be done. If the system uses armor as reducing damage, though, then attacks which miss, and attacks which hit but do no damage against armour, will be identical. They'll both show damage 0. Probably the best way that feels most like a tabletop game is to have the attack roll print out a button along with the attack result. Players can click the button to roll the damage, and it prompts them for the damage bonus through a dropdown query.