The skill tree from the main game is already complete in the DLC, don't worry.
But you'll have 7 new skills to purchase, they are all tied to the new "fear" aspect of the gameplay.
Like in the main game, you'll unlock skill points by earning XP in many ways (completing story missions or loyalty activities, kills...).
You'll need to choose any 4 skills to unlock the cheevo.
On your way to 100% in everything the DLC has to offer you'll get more than enough XP to purchase all 7 skills...
So it's kinda unmissable as long as you pick your skills at one point...