I wouldn't male a starter with dry yeast. If you think you need more (I'm not sure you do), just pitch another pack.
This advice only applies to the initial pitch. It doesn't apply in this case. Here, you want to pitch yeast that are in a state of high krausen. This way, their metabolic pathways are firing on all cylinders and ready to go to eat the remaining sugars in your wort.
If you just sprinkle the dry yeast in, you're adding those yeast to a very stressful environment--sugars and nutrients have been depleted, and CO2 and ethanol are present. I doubt those cells would do much, if they even survive.
You are just trying to eke out ~7 more gravity points of attenuation. Whether you use dry or liquid yeast, you want it at high krausen. A starter is the only way to achieve this.
If this were me, I'd make a 1-pint starter using any ale strain I had on hand, and pitch at high krausen. If it was dry yeast, I'd use perhaps only half the sachet.
Or you could just wait it out and see what happens. There's no harm in this, even if it takes many weeks.