If you want to model an external framework, simply draw it as it's own layer and draw in it's dependencies with the other layers that consume it. The lines might start to draw over top of other layers so you'll have to lay it out on screen to show it as
cleanly as possible.
The layer diagram is not UML compliant and at this point there is no support for associating layers with the UML model elements. However, you can associate any layer with the physical elements in solution explorer (projects, namespace folders, and project
files). Simply drag and drop them onto the layer they belong to. You can then start working with the architecture validation tooling. I stopped short of showing this becuase I wanted to focus on the UML tooling. You can find a great starting tutorial here: