Forgive me all you c# users but I do know know how to do this with a filestream off the top of my head, so here it is using the dreaded VB libraries...
[VB]
Dim FileNumber As Integer = FreeFile()
FileOpen(FileNumber, ExistingFile, OpenMode.Append)
Print(FileNumber, AppendedText)
FileClose(FileNumber)
[/VB]
ExistingFile would be a string that contains the name of the file you want to append to. AppendedText is... you guessed it, the text to append.
Maybe someone more familiar with the FileStream class can show you a "better" way to do it. Ive been accessing files the VB way for years and some old habits die hard.