The idea would be simple. You already succeeded in having the player running while hitting the ball when at full speed. I don't know the specifics, but I imagine that in this case, you have made a script to dissociate the upper body from the legs in order to mix two different animations.

Can it be possible, then, to do the same to match the ball's height ? I'm saying just 3 differents ones :
- low balls (knees folding)
- normal balls (animation unchanged)
- high balls (little jump maybe ?)
Is it difficult to implement ?