A hammer will only stretch it more. Most procedures for shrinking metal apply to damage where the stretch is limited to a local area and this are is heated and between the surrounding metal influence and careful hammer and spoon work the red hot metal can be brought back to the original shape. You have a piece that now is too big. Can you jack the sill out a bit? You situation is more like dad said they did on-board Navy ships. He said they would heat the deck up and then threw water on it. So I tried it without any luck on a multiple hit van door. I didn't have the rest of the ship helping to force it back into shape. The workshop manual shows a pile of wet asbestos with holes in and heat is applied through the small holes. I think the theory was to have the cold steel surrounding the hot steel to coax the hot steel to occupy less space. I think the other trick is to have a way to move the panel into final shape so that the internal stress war will take place. You might want to try heating the entire area and then have a way to clamp it into shape. Then as it cools it might lay flat. Sorry no successful trials on my part. That is just the theory. I actually only had the one need many years ago and only recently read about these other approaches because some day I plan to tackle those doors again. I actually got the door fairly straight using wet towels, heat, and careful hammering but when I closed in the holes the previous owners drilled to hold the putty, they got all wavy again, so I just put filler on back then.