Great! Just click “I have a domain” on the last step, instead of buying a new one.
Google prefers older / aged domains, as it is less likely to be a “spammy” junk website.
If you already have hosting, note that our system has massive code under the hood, and furthermore, you really have an “engine” instead of “a complete car”. For more information, if that’s you i.e. you already bought hosting, and can’t just cancel it and use ours, then see http://swiftwebdesigner.com/hosting as to why we require our sites to use our hosting / server.
Also, people often use multiple domains for marketing and seo (search engine optimization) purposes. We recommend you set up something like MainBrand.com then build a page at MainBrand.com/my-product, then if you have another domain, say, myproduct.com, then point that via “301 redirect” to MainBrand.com/my-product. In this way, multiple domains can “pass through” “pagerank” (i.e. “SEO juice”) to your main “money site” i.e. the main site on which you do business.