You've got to do the following:
Create a new column with a new skill on skillname-e
Create a new column with a new skill on skillgrp (Including it's icon)
Make a buff on skills folder inside an existing one or create a new xml and edit the stats you want.
Add the custom skill in the dagger inside weapon table.
You're done.
Else; Create a hidden custom passive skill which is not added on skillgrp and skillname-e and add the stats including the line <using kind="dagger"> and add it on the character's skills so everytime he equip any dagger he will get the same effect.