Add one subclass to admin

Admin/Admin.php

@@ -1075,6 +1075,16 @@ abstract class Admin implements AdminInterface, DomainObjectInterface
         return $this->subClasses;
     }
 
+    /**
+     * {@inheritdoc}
+     */
+    public function addSubClass($subClass)
+    {
+        if (!in_array($subClass, $this->subClasses)) {
+            $this->subClasses[] = $subClass;
+        }
+    }
+
     /**
      * {@inheritdoc}
      */