I wouldn't see why you couldn't just steep the chocolate malt at 150 for like 30 mins, cool and then dump into the primary. I'm sure it's not the best practice, but should get the flavor in there. You could also probably boil and add some dry malt extract to get the gravity up a little bit.
I had an AG oatmeal stout that I royally messed up (BIAB). Whatever you do, DO NOT try to compensate for the gravity by adding dextrose, and then try to compensate for the lack of thickness by adding "oatmilk" (boiled and strained oat liquid). While the beer didn't have a terrible flavor, it was as thin as water and tasted like burnt toast. I tried to drink as much as I could, but ended up dumping it to make keg room for something I didn't ruin!
Good luck with what ever you decide to try! And everyone else, please correct me if my suggestions are bad! Brewing's a learning experience and I'm never too proud to say my idea was bad.