No, you cannot. Engraving is available only when you order through the online Apple Store. Otherwise, you'll need to take the iPad to an engraving shop.
Think carefully before you have an iPad engraved. Engraving usually just decreases the potential resale value, and since most people keep their iPad in a case just gets covered up anyway.
No, you can't get the engraving done at an Apple store. As the previous poster noted, the engraving can noly be done when a product is ordered online. The engraving is actually performed by a high-power laser at the manufacturing facility. Engraving performed using a mechanical tool - at a jewelry shop, for example - might cause internal damage. Another option is to use a third-party company that also uses a laser. Check out www.etchamac.com.
Story: I had a workmate who's engraved iPad was stolen.
He had it replaced...either he bought one himself or through our work insurance can't remember which.
3 months later the Police from another city called and said they had recovered his stolen iPad...identified as his through the engraving. Don't know how they found him.
Long story short...he now has two iPads.