Convert from System.IO.FileStream to InStream

Hi there

I'm trying to load the FileStream of a file on the client with System.IO.File.OpenRead() into a InStream but I get the error:

Unable to convert from Runtime.NavDotNet to Nav.Runtime.NavInStream

FileStream := ClientFile.OpenRead(ClientFilePath);
InStr := FileStream;

I tried with different .NET Stream types and with COPYSTREAM but no luck so far... :-(
What am I missing here? Does anyone have a working example?