This method changes the position of the attached stream relative to its current position by the amount specified in
offset.
A negative number can be used to move backwards form the current position.
var fileStreamObj = shell.serviceManager.basics.fileStream;
var streamWriterObj = shell.serviceManager.basics.rawStreamWriter;
fileStreamObj.openForWrite("c:\\temp\\test.txt", false, true);
streamWriterObj.stream = fileStreamObj;
streamWriterObj.seekRelative(4);
streamWriterObj.writeDouble(20.5);
fileStreamObj.close();