Hi,
I imagine you could do this by changing the email-form.xpl Erik was pointing to his previous message. You can see how this is done for the "To:", and do something similar for the "From:". One potential difficulty is that this code is written in XPL/XSLT, and if you are not already familiar with those languages, you might want to learn about them first.
Alex
--
Follow Orbeon on Twitter: @orbeon
Follow me on Twitter: @avernet