If the body is in the showcase you can have it routed for a humbucker. If it’s already in your possession, I don’t think they would do that for you.
This is a good example. Looks like someone ordered a showcase body and either canceled their order or returned it, but it was re routed for a humbucker
http://www.warmoth.com/Showcase/ShowcaseItem.aspx?deals=true&Body=2&sort=price&i=PT7496#.XIbbg6ROnDs