There is no way to specify that the server (from a client), but you can know if a server will transform or not through its nip96.json file.

In this way, you can choose which server to use, the one that best suits your preferences.

Thinking about it, it would not be difficult to implement. But then the server could deny it to you depending on whether you have access to that service or not. 

 Ok I will check existing servers to find one that doesn't transform. Consider adding that as an option per upload.