How can I switch from Sitebuilder to Dreamweaver without having to start from scratch?

Sitebuilder and Dreamweaver are two different tools for web design, which makes them incompatible.
You can still download the website you created using Sitebuilder to your local machine using some FTP client, edit the site with Dreamweaver, and re-upload it but there is no guarantee that everything will work as it is supposed to. When designing a site it is best to pick a web development tool and stick with it throughout the whole process of designing, developing, and maintaining it. So if you think that Dreamweaver would be your final choice as your site development tool our advice is to start building your site from scratch and when you are ready just replace your current site with the new one.