That might work,but I'd say it's unlikely. Most Dobs aren't made for prime focus with a DSLR - you can't get the camera's sensor close enough to the secondary mirror. The extension tunes are the opposite of what you need.
Luckily there's a cheap workaround - a 2X Barlow T-adapter. I used my own D750 with my 8" Dob with just this one:
http://shop.australiangeographic.com...th-barlow.html
I got fantastic results on bright subjects like the Moon, the Sun (with a solar filter of course) and the planets. Managed to photograph the ISS!! I'm still pretty chuffed with myself for that effort.
You'll need a Nikon T-ring as well if you don't have one already.