VB.NET Insert string into file

[DeMoN X]

I want to insert text into a file, However this text needs to be on a certain line, and any new text created by the program would need to be added to that line however existing text will not be replaced.

For example when ran for the first time :

Line 1 |
Line 2 |
Line 3 | *Insert string here*
Line 4 |

When ran for the second time :

Line 1 |
Line 2 |
Line 3 | *Insert string here**Insert another string here*
Line 4 |

Any ideas?