No hay ningun problema es escribir strings o chars en un stream, no hace falta convertirlos en bytes. Por ejemplo:
Código Delphi
[-]
var
Stream: TMemoryStream;
S: String;
C: Char;
Buffer: array[0..255] of byte;
begin
S:= 'Hola mundo';
C:= ',';
FillChar(Buffer,Sizeof(Buffer),0);
Stream:= TMemoryStream.Create;
try
Stream.WriteBuffer(PChar(S)^,length(S));
Stream.WriteBuffer(C,1);
Stream.WriteBuffer(Buffer,Sizeof(buffer));
Stream.SaveToFile('Prueba.txt');
finally
Stream.Free;
end;
end;