[dogscoff]

The total mass of all the asteroids is less than that of the Moon.Of course, we dont' know anything near that much about the Oort Cloud or the Kruiper Belt...

Still, I'm not sure you can make much of a planet out of it all.

Well, make it hollow then (think core-mount weaponry/ sound-system/ insert preferred tech-fetish here). Or make a Culture-esque Orbital (very material-efficient: Apparently you could make 10 Orbitals out of one Earth, so a moon-ish pile of rock should be plenty.)

If all else fails, we'll just "borrow" pluto and neptune. No-one will miss them. The Dofscofia project must go ahead!