Freezing an object and keeping it's velocity

Consider the following: I have a cube, which has a function called Freeze. It sets the cube rigidbody to kinematic, and when called again, it unfreezes said cube. I was trying to implement a way for the cube, after being "unfrozen", re-adding it's speed from BEFORE it was frozen, to give and idea of it being frozen in space-time. However, the code I wrote does not work. Is there a way to do this?

Code I used (C#):

void Freeze() {

     if (isSelected) {  
         if (!rb.isKinematic) {
             retainedSpeed = rb.velocity;
             print (retainedSpeed);
             rb.isKinematic = true;
             cubeMesh.materials [1].SetColor("_EmissionColor", freezeColor);
         } else if (rb.isKinematic) {
             rb.isKinematic = false;
             rb.AddForce (retainedSpeed);
             cubeMesh.materials [1].SetColor ("_EmissionColor", defaultColor);

         }
     }
 }