I don't have an optic on mine, but I do have the quad rail fore end (gen 1), so if I was feeling the need, I could slap on either a micro dot, or Eothech, with quick release mounts so it could still fold. Any permanent mount optic would make it not able to fold which would defeat the whole purpose of this gun inmo. Obviously Red Lion makes those rotating fore ends, but to me they are too bulky once you add an optic, especially since the optic then hangs off the side when it's folded. I look at the Sub2k as a concealable backpack, or truck rifle so anything that adds bulk is a non starter for me. If you want/need an optic though, I'd just get a PA micro on a QR mount and call it a day.
Edit to add: I see you didn't mention what gen you have. If it's a gen 1, you will have to get a quad, or some other method of rail attachment, but the new gen 2 has that nice fore end with the ability to attach rails, so that would be easy. I may end up selling my quad and getting a gen 2 fore end just because it's way lighter, and I don't utilize all the rail space anyway. I just got it as a cool accessory when I first got the gun and didn't have many rifles to hang crap on at the time.