Yes, there is a way to create Sandcastle documents inside Visual Studio: DocProject
To quote the project website:
DocProject manages Sandcastle support using Visual Studio 2005/2008 and MSBuild. Choose from various project templates that create compiled help version 1.x or 2.x for all links to projects. DocProject facilitates the administration and development of project documentation with Sandcastle, allowing you to use the integrated Visual Studio tools to customize Sandcastle's output.
Mark