After explaining how to navigate the files of the Solution Explorer using the IVsHierarchy interface (which can be done from a package or from an add-in), as I stated I am going to show how to get the icons of the Solution Explorer items (solution, projects, files, folders, etc.):
HOWTO: Get the icons of files in the Solution Explorer
I have done it with a package rather than with an add-in, but once you understand the technique you can adapt it to your extension.
Once loaded, the package provides a toolwindow that shows a treeview filled similarly to the Solution Explorer.