You can also assign a quiz to policies in Approve or Published stages.  However, to do so, you must first move the policy to Draft.  To assign a quiz to the Draft policy, go to either the Quiz Center or the Policy Center:

In the Quiz Center, either create a new quiz or select an existing unassigned quiz to assign to the Draft policy.

OR

In the Policy Center, select the policy from the Draft Policies list to edit.  On the policy edit screen, click Quiz to assign an existing quiz to the Draft policy.