Good question, and we don't currently have this documented.
The Business Application templates include the ASP.NET features, along with navigation and theming. If you want the navigation and theming too, then the best approach would be to start with a BAT and then include your existing code and/or projects.
If you don't want the navigation and theming, then it's probably easiest to create a solution from a BAT and then copy files over to your existing solution. These files would include: the web.config portion(s) dealing with ASP.NET; all files in the web project's Services and Models folders; all files/links in the SL project's Models folder; and the AuthenticationManager in the SL project. If you do want the Login-related views also copy those over, but you'll then need to selectively include some of the code from the Controls and Helpers folders. You'll also need some of the Resources from both projects too, since they're referenced by other files.
Please let us know how it goes or if you run into any issues.