We may be getting closer!
Just recently, a S&W rep said the following in a thread on another forum:

"We are in the process of adding M&P 10 parts to our website and customer service phone support. As of right now I am not sure what exact parts are available but you can call customer service and request parts at: 1-800-331-0852 "

"The barrel threading of the M&P10 is compatible with the DPMS barrel threading" made in reference to attaching forends

"As of right now, you cannot purchase barrels directly from S&W. I am working op getting that changed, but it may take a little while. I will post once we get these items available for purchase."

Am hoping that it would be possible to buy a S&W barrel extension or remove one from a S&W barrel and install it on a DPMS barrel, or have a custom barrel made.