The name of your question is slightly contrary to the text of the question.
If you want to include plain text, you can try something like this:
#+begin_src sh :exports results :results output cat your_file.txt #+end_src
I have used this in the past to add file contents to a web page through HTML export. The results of the src block, in this case, are set in the block in HTML. Not sure what TaskJuggler does.
You can also do this with the macro #+include : http://orgmode.org/manual/Include-files.html If you want to include certain parts of #+include 'd, use the :lines parameter, as indicated in the link above.
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