If I'm following your question right, then this should be pretty simple. When logged in as the user that the account is for, simply type
echo "real_user@isp.com" > ~/.forward
and then
chmod 600 ~/.forward
This should do what you're asking for.
P.S. This will work assuming sendmail is set up correctly and running. If that isn't true, say so, and someone can post on how to set that up right.