Let's start by figuring out which fields you'll need to modify.
In the Develop menu, make sure Enable Data File Debugging (at the top) is turned on.
Then, add a test Katana, right-click on that weapon, and choose "Show Debug Fields for Katana".
Now, in that list, filter for "Damage", and then for "Armor Piercing", and tell me which field stores the value for each of those.
Now, can you name a weapon mod that makes a modification to the stats of its weapon? Make a copy of that, look at its "Eval Script" button on the upper right-hand side, and paste that script here. Then, I'll help you modify that script to make the changes you want Dikote to make, instead of whatever changes that mod is making.