I don't know how to append new text to an existing file, but what you can do is collecting everything you want to have in the Textfile in Clipboard.
I'm using leo's tool Setclip.exe for this:
[url]SetClip]
Here's the basic Code for collecting the full paths of all selected files in Clipboard which you can change to your needs (e.g. Select ALL in the second line):
@runmode hide
"[Path to Setclip.exe]\SetClip.exe" -a {filepath|noterm}
Before you start your Collection you should clear the Clipboard with this (put it to the RMB of a 3-way Button):
@nodeselect
@runmode hide
"[Path to Setclip.exe]\SetClip.exe" -e