If you’re modifying the animation in-engine, lower the root bone or pelvis bone to where you want it, being careful only to change the z axis value. Near the top of the screen is a big plus sign button called Key. Pressing that or the hotkey S will create an additive layer track for that bone. Do that, then hit the apply button near the top and save it. When you play the animation in the engine, you should see the change.