How can I write data from a C# app into GE Proficy Historian

I would like to use GE Proficy Historian to archive data from instruments that are not connected to a PLC. Can this be done using c#?

You can use the Historian c# User API wrapper to access and update Historian from c#. There is information available on it in the Historian documentation that installs with the client tools. Updating Historian with data from non-PLC connected devices is counterintuitive though, as you have the data elsewhere (I assume) and Historian is designed to log activity from PLCs and similar devices.

It will probably be more difficult to read data from non-PLC connected devices into c# than to write data to Historian.