EDIT: Saw you wanted to use AddForce, Im not sure, but I guess this same principel should work with that also. BTW: This is done in C#

Tried to make a quick example, I think this should do the trick, Fiddle about with it, to get it to fit what your trying to do.

using UnityEngine;

using System.Collections;

public class testMovementScript : MonoBehaviour {

public Rigidbody theBody;
public float moveSpeed;
public float rotation;
private Vector3 moveDirection;

// Use this for initialization
void Start () {

}

// Update is called once per frame
void Update () {
    transform.Rotate(Input.GetAxis("Horizontal") * rotation * Time.deltaTime, 0, Input.GetAxis("Horizontal") * -rotation * Time.deltaTime); //This is where you rotate your GameObject.

    //This is where you move the GameObject about.
    moveDirection = new Vector3(0, 0, moveSpeed * Input.GetAxis("Vertical"));
    theBody.transform.Translate(moveDirection);
}

}

I'm not sure if I understood what you want to do. If you want a force that points into another direction all you have to do is rotate the force before you add it. If you want to change the direction of the momentum the sphere already has you will have to rotate the spheres rigidbody.velocity accordingly.

You need to track the forward direction yourself as a variable and apply a constant force in that direction to maintain movement in the correct direction. Use a highish drag value to prevent the ball from picking up speed over time. When you press the left and right keys rotate the forward direction and apply a force relative to the forward direction.

Placing the sphere in a container game object will help you with keeping the correct forward direction.

It's a bit of a round about way but movement like you describe isn't very realistic.