Added IK bone messing up excisting animation I have downloaded and imported a model from Mixamo. It doens't import right, but after some experimenting I managed to get the existing bones right. 

The problem is that I want to add IK to the armature. But adding IK bones messes up the existing animation, because they don't follow along.

I just want to edit a part of the animation with IK. Does anyone have the knowledge on how to do this?
You could deactivate the IK when needed: Click on the eye icon to deactivate, put your cursor on it and press i to insert a keyframe. If you want to reactivate, same thing, click on the eye to reactivate and press i.