A Box object is very different from a Pen object. It doesn't make sense that a Box would be a specialized version of a Pen. This is like saying a polar bear is a specialized form of a footbag. What you should do is simply use a Pen within the Box class as a local variable. For example, if the box needs the pen to draw itself, then perhaps in the draw method include something like
|myPen| myPen := Pen new. Then you can use the myPen instance of Pen to do what you need to get done.