Yes, But only if all the asteriods were moved to a different orbit, well away from the disrupting gravity of Jupiter which keeps the asteriod belt well stirred. There are enough asteroids to make a moon about 4% the mass of the earth's moon. read more
No. There is not enough gravitation among the largest asteroids to produce a major planet in the asteroid belt. The action of Jupiter and Saturn prevent the formation of a planet in the asteroid belt. read more
And if you think about it, there's actually not that much material in the asteroid belt. Ceres, a object that has not even 2% of the Moon's mass, is 25% of the total mass of the whole belt. Coupled with the fact that this mass is so spread out (a 'close encounter' could be a couple of hundred thousand kilometers away from any two asteroid, with collisions really unlikely) planet formation is physically impossible. read more
Approximately 1/3 of the asteroid belt's mass is in the dwarf planet Ceres. The Kuiper Belt has more mass, estimated at 1-2 orders of magnitude more massive than the asteroid belt (which still is only comparable to Mercury's mass), but is spread out over a vastly larger area. read more