If you are doing it for single file as you have mentioned you can try this:
System.IO.File.Move(Dts.Variables["strSourceFile"].Value.ToString(), "D:\\Temp\\output.txt");
Where the first parameter is your variable name.
If you are looking for expression and I am assuming that your file name is alwasy XXXX_output.txt where XXXX is any character than you can try
System.Windows.Forms.MessageBox.Show(Dts.Variables["strSourceFile"].Value.ToString().Substring(Dts.Variables["strSourceFile"].Value.ToString().LastIndexOf("_")+1,(Dts.Variables["strSourceFile"].Value.ToString().Length-Dts.Variables["strSourceFile"].Value.ToString().LastIndexOf("_"))-1));