It sounds like you're asking if you can put a mountain bike tire directly over the wheel + tire of a road bike.
(please correct me if I am wrong)
There are a few things to account for.
It is not very safe to put a tire over another tire, as most tires are slick and the inner tire may not gain traction on the inner tire.
It is not likely that the mountain bike tire will even fit over the road bike tire, as even the largest mtb tire will most likely not fit over even the smallest road bike tire size.
In the end, it seems that doing this is not possible and even if it worked, it would still be unsafe.
Edit - It seems I have missintepreted the question. with the diameter and width of mountain bike tires, it is practicaly impossible to fit even a 29er on a road wheel, not because of the diamter, but instead the thickness of it.