You can't do this in C# since C# does not have a preprocessor, and the C# compiler, although it supports some syntax that looks like preprocessor syntax, is handled by the compiler itself, and the compiler doesn't support include directives.
Also, if even one file actually requires 20 using statements it sounds to me as though you have put too much responsibility into one class. Perhaps this would be better served by refactoring and restructuring the codebase, thus reducing the number of required using directives instead?
Or, you could look at it from a different angle. You say that these 20 using directives all "belong logically together", why then 20 namespaces? Sounds to me as you've spread out some functionality into too many namespaces.
So either way I would look at restructuring the code.
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