You can make the classes internal but this only prevents anyone outside of the assembly from using the class. But you still have to make a separate assembly for each namespace that you want to do this with. I’m assuming that is why you wouldn’t want to do it.
Getting the C# Compiler to Enforce Namespace Visibility
There is an article here (Namespace visibility in C#) that shows a method of using partial classes as a form of “fake namespace” that you might find helpful.
The author points out that this doesn’t work perfectly and he discusses the shortcomings. The main problem is that C# designers designed C# not to work this way. This deviates heavily from expected coding practices in C#/.NET, which is one of the .NET Frameworks greatest advantages.
It’s a neat trick… now don’t do it.