How to Create Additive Animation

I'm using the animation preview package and I want to edit an existing animation.
When I play the animation on my character, the legs are too close together and the arms are too far forward.

How can I make these changes to the animation without overwriting the animation in the process?
I want to be able to make an additive tweak without for example putting a constraint on the leg that will make it so I have to redo the leg animation.

I've tried the asset UMotion Pro - but when I use that, the animation breaks (changes) in a weird way when I import the clip into a umotion project.