visual studio 2015 - Add reference to an existing assembly (source not included) -

I am evaluating the possibility of migrating to the next MVC3 application to the next MVC6. I have to import a commercial assembly except for one thing, for the migration (session, security, etc.), which I use to export data in Excel, Word and PDF, I do not have a source code, And there is no way to add references in the Visual Studio 2015 solution or using NuGet, except those projects. I do not have a NuGet package, because this business assembly is from pre-newgate time, so, how can I add a reference to this assembly? MVC6 project ?, and BTW, I am not interested in core-CRR now, then multiplex Support is not required.

Thank you.