Yes, not visible to end users, but just to have proper naming convention. Is it possible?

dont' think so, why would you want to change them ? They are not visible to your end-users.

actually you can (man nas_server). I would advise you not to do it because all documentation references datamovers as server_2 or server_3.

Like dynamox said I would not recommend to change them if your only reason is naming convention

Thank you very much.