There are a couple of ways you could go about this.
1. If you are inputting the amount of force used to push the ball, you can have the ball store that number as a vector3 and when it collides with the other ball, get that vector 3, stop the force from the old ball and apply it to the new one.
2. If you dont store the force in a variable, you could use get velocity to get the vector 3 and do the same steps above with getting and setting the value from the old ball, to the new one.