There is not a direct way to convert a SiteAssist site to a SiteAssist Professional site. In order to bring SiteAssist Pro code up to modern standards, the architecture has really changed, prohibiting us from converting old code to new that easily. Like Eric mentions, SiteAssist Pro is really a new product, not just a newer version of SiteAssist.
You can, however, use the New Layout wizard to import a DWT (DWT must be copied into your defined site in order to select it in the wizard), then you would need to build out your sub-pages from there.