Okay, I think I just figured out how you can do this. I've used this kind of mb software before, but never figured this out!
If you look up at the blue bar right on top of the first post in a thread, on the far right you'll see "Display Modes". Click on the arrow, and choose "Switch to Hybrid Mode" in the drop down meny. This will give you a view of the thread like the old board on top, with the content of each post on down the page. If you click on the post (in the top "threaded" view) that you want to reply to, your reply will show up under their post rather than at the end of the thread.