I would like to be able to do this as well.
I will let you know once I figure it out. I was hoping someone would let me know on here right away.
I would also like to remove "Attach Files" field completely since photos or other files can be attached in the "Post".
Well you can do that using jquery.
Here's the theory that i have not tested.
Use css to hide the POST field , then on use click the add discussion button , use jquery to insert anything into the post field textbox (to prevent Ning from restricting user to submit with an empty post field).
After that you will just need to use css again to hide the submitted discussion post body.
I do not want to hide it, I want to make it optional because sometimes people need to add question details.