No, A mountain bike has specific features that make it a mountain bike suited for off road, and a road bike has features designed for the road. A mountain bike has suspension for a start, and the forks are too far apart from each other anyway, so different forks would be needed. There is so much on a mountain bike that would need replacing (pretty much everything) that it is just logical to buy a new road bike. However, I would recommend getting a cyclo-cross bike, which is a very similar design to a road bike, with only small differences. If you only want to get one bike but want to do road and off road, then cyclo-cross bikes are perfect. They have thin tyres, although not quite as thin as road tyres, but thinner than mountain bike's. The tyres also have nobbles on for grip in off-road, and it doesn't have suspension so road wheels can easily be put on the bike to make it a cross bike converted into a road bike. It has the same aerodynamic design as a road bike and the only other difference really is it has a wider gap in thr brakes so mud doesn't get stuck as easily. Hope that helped.