Is there any standard way to transform object to different object?

I'm having a special case where I have implemented builder pattern. One of the variable is of type ExternalClassA, Where as the parameter passed to builder is of type InternalClassA. Currently I have used a method something like below:

Class Parent {
   String variableX;
   int variableY;
   ExternalZ variableZ;
   public static builder {
   ....
   ExternalZ varz;
   /* .. builder logic */
   private builder withExternalZ(InternalZ internalZ) { //This is the transform method I'm using currently
   this.variableZ.getSomeObject().setX(this.internalZ.getSomeObject().getX()); //Similar kind of lines making my transform method look ugly
    }
}

Is there any way to make my withExternalZ method look simpler? Currently I'm out Mapper pattern and Java8 Function (As explained in http://ift.tt/17MYsBW).

If anyone has better and simpler approach, kindly share.