Is there a way in VB.net to append text in a textbox, but leave the original text?

In a VB.net program, I have a textbox for notes.  The note will be pulled from a text file.  I'd like it so the user can read the note, but not modify it.  If the user wants to add to the note, he can type below it.  Is there a way to have read only text in a textbox, but allow the user to add text below it, or do I need 2 textboxes to accomplish this?  I would like to keep it to one textbox if possible.

Once the user adds text below the current text, then I have an "Add Note" button that I would like to have add the users text at the bottom of previous read only text and now the additional text would also be read only.  Users could keep adding more and more notes to the textbox, which get saved to a text file.  Is this possible to do with one textbox, or do I need two?