How do I import a package from the local filesystem using dub?

I have a project that uses dub. I want to use an external file vendored into my project, as a dependency. How do I do this? I don't want to have it in my project's source/ dir. I don't want to add it as a dub managed dependency, but I do want to be able to just import xxx.

The package is this one: https://github.com/gianm/d-json , it does not use dub or have a dub.json project file.

Alternative thing: make a third_party directory, put the file in there, then add that to the sourcePaths in your dub config (you'll probably specify both ["third_party", "source"] since the default source will be overridden if you don't list it too.