assuming the text you want to add is simple enough and they are all txt files, navigate to the directory where they are and create a run.bat file with the following contents
for %%f in (*.txt) do (
echo your text here >>%%f
)
thanks should be given to https://stackoverflow.com/users/37923/mark and https://stackoverflow.com/users/2666/jim-buck
Naturally, I would recommend a test first before going through your thousand files.