The MSBuildWorspace is built around the concept of handling MSBuild solutions (.sln files) and their respective projects (.csproj, .vbproj).
Adding new projects and documents to this workspace is not supported.
string solutionPath = @"C:\Path\To\Solution\Sample.sln";
MSBuildWorkspace workspace = MSBuildWorkspace.Create();
Solution solution = await workspace.OpenSolutionAsync(nancyApp);
var allDocumentsInSolution = solution.Projects.SelectMany(x => x.Documents);