How to Colonize an Asteroid
While we would like to end up with a number of small ponds or lakes around the equator of the asteroid, it will a first be necessary to flood large areas of the interior sphere to provide a home for the algae which will eventually transform the atmosphere and begin building the soil.
Water is available in a few different forms in the asteroids.
The best source for water might be cometary ice from the dust covered cores of dead comets which are known to orbit with the asteroids. Bringing hunks of these cores into the asteroid and letting them melt would also add a great deal of volital organic compounds to the environment - helping to build the soil and add to the chemicular diversity of the colony. Of course, we should make sure that there are not hazardous levels of some materials - such as chlorine gas and the like.
We may also be able to extract small amounts of water from the material being removed from the center of the asteroid, but a metallic object is not the best place to find water. More likely is that water will be found in bound up in compounds in a carbon based asteroid. We will need one of these asteroids anyway to procure several other chemicals and elements.