Well, I guess you'd have to register at least two domains, so you can publish a total of three sites (one to the free sub domain). Of course, registering paid-for domain names to export to other externally hosted domain names kinda defeats the purpose of the export function. You should probably just move your external domains to be hosted under your Website Maker account. (Check the help/FAQ section on the online services website for more information.) This way, you only need to manage and pay for one single hosting package.