--- workflow.module
+++ workflow.module
@@ -1238,6 +1238,9 @@ function workflow_allowable_transitions($sid, $dir = 'to', $roles = 'ALL') {
     }
   }
 
+  // Allow other modules to alter the allowable transitions.
+  drupal_alter('workflow_allowable_transitions', $transitions, $sid, $dir, $roles);
+
   return $transitions;
 }