AspNet "Visual studio Code" for Mac, how to add a local DLL dependency in project.json manually or using DNX, DNU?

Question

There is third party DLL stored as a normal something.dll file (i.e. it is not a part of the repository, and it is not published using the repository) and sample code for this DLL, Program.cs. Whole project is simple console app, generated using yo.

How to add a dependency to this DLL to project.json?

I would like to add the dependency manually to project.json with reference to the local file, or I would like to know how to add the file to the local repository (which is stored in .dnx folder, I suppose) and then use it in package.json.

My ASP.NET Visual Studio Code installation uses dnx and dnu. Thank you in advance.