How to add method to partial in different namespace?

In .NET 3.5, C#, if I have a static partial class defined defined in namespace nameA, how can I add a new method to this partial if I'm then in namespace nameB?

namespace nameA{
public static partial class TestA{public static string stringA = "testingA";}
}

namespace nameB{
//this doesn't work
public static partial class nameA.TestA{public static string stringB = "testingB";}
}