Out of curiosity, what transport layer are you interested in?
The simple answer is no, this is not possible in C#.
A longer answer is there are 2 theoretical possibilities, but unless you're ready to basically maintain your own fork of gRPC, I certainly wouldn't recommend any of them:
1. you could write your own transport layer in C/C++ (expert level stuff), add it to C-core library and use it from the C# wrapping layer (after building your own custom nuget)
2. you could use Grpc.Core.Api to implement the gRPC C# public API yourself and create a custom C# "channel" that will do whatever you want. (this is basically what the grpc/grpc-dotnet project has done). Note that this approach basically means you're rewriting the entire gRPC implementation yourself, so definitely not recommended.