In the D&D Q&A Web column "8/31/07" the Sage weighed in on this very question:
Can a warforged character attach an armor augment crystal to his own body?
Yes, but only if the warforged A) has some sort of composite plating (either the base version or an augmented version from a feat such as Adamantine Body), and B) that plating has a sufficient enhancement bonus to accept the crystal.
For example, a warforged whose plating has a +1 enhancement bonus could attach a least or lesser augment crystal to his own body. Without at least a +1 enhancement bonus, the warforged couldn’t even use least armor augment crystals, since his body isn’t considered a masterwork suit of armor.
(Edited slightly for readability.) The Sage in this exchange is Andy Collins.
Although Sage Advice should often be viewed as just advice rather than, for example, errata, the Sage's opinion on this topic so happens to mirror my own. That is, the Magic Item Compendium on Augment Crystals says, in part, that "[a]n augment crystal… provides a magical effect when affixed to a… suit of armor (or any other magic item that grants an armor bonus to AC)" (221). Thus, once the warforged's composite plating's been made magical so that it has a magical enhancement bonus of +1 or more—becoming, in essence, an irremovable magic item—, the composite plating's become suitable for attaching a least or lesser augment crystal. (It's not until the composite plating has a magical enhancement bonus of at least +3 that to the composite plating can be attached a greater augment crystal.)
With that in mind, the warforged racial trait composite plating, a natural ability, in part, says, "Composite plating can gain a magic enhancement bonus and magic armor properties as armor can, using the Craft Magic Arms and Armor feat" (Races of Eberron 8). As every typical warforged can pay to have its composite plating made magical, it appears that even a typical warforged composite plating is virtually masterwork enough to be able to become magical, so this DM would likewise allow a typical warforged to attach a least augment crystal to its plating immediately without needing to have its composite plating somehow modified. However, ask your DM as she may rule differently.
Paying a creature to make the warforged's composite plating magical costs no less and no more than making an existing suit of masterwork armor magical. The only differences between the two processes is that no actual suit of armor need be provided and that the warforged must be present throughout the magic process. But, as the typical warforged doesn't breathe, eat, or sleep, it isn't like the warforged's presence is a big imposition on the creature doing the magic.