I have a testing automation framework written in C# with unit tests written in NUnit (not MSTest). I created a new Visual Studio Load Test
Until now there is no package or workaround, other then converting your NUnit tests to MSTests, to add your NUnit tests to a LoadTests. The same problem also applies for XUnit test. I hope MS will enable the possibility to add XUnit and NUnit tests to a LoadTest.
A solution is that you can convert you NUnit test: http://www.stormbase.net/2010/01/12/converting-from-nunit-to-mstest/