Add a new line at a specific position in a text file.

I am trying to add a specific line of text in a file. Specifically between two boundaries.

An example of what it would look like if I wanted to add a line in between the boundaries of item1:

[item1]
2550 coins 995 200000 7
2550 coins 995 200000 7
2550 coins 995 200000 7
2550 coins 995 200000 7
2550 coins 995 200000 7
//Add a line here in between the specific boundaries
[/item1]
[item2]
2550 coins 995 200000 7
2550 coins 995 200000 7
2550 coins 995 200000 8
2550 coins 995 200000 7
2550 coins 995 200000 7
[/item2]
[item3]
2550 coins 995 200000 7
2550 coins 995 200000 7
2550 coins 995 200000 7
2550 coins 995 200000 7
2550 coins 995 200000 7
[/item3]

This is what I have tried so far, however Its nowhere near correct. It keeps saying that the file is being used by the reader so it cant be edited by the writer, when I did get it to work it cleared the entire document.

public void createEntry(String npcName)
{
    String line;
    String fileName = "Drops.de";
    StreamWriter streamWriter = new StreamWriter(fileName);
    StreamReader streamReader = new StreamReader(fileName);
    line = streamReader.ReadLine();
    if (line == ("[" + npcName + "]"))
    {
        streamReader.ReadLine();
        streamWriter.WriteLine("Test");
    }
}

I would also like to know how to write lines at the end of the document.