As a workaround, you can add the &Login&Username=yourname&Password=yourpassword parameter to your URL but then these parameters are displayed to the user of the exported content.
To get around this, I created a separate login project using a URL like:
I set this with a level limit of 0.
Then I created the real project using a URL like:
To download the content you run the first project to log you on to your Notes/Domino server, then you download the real project. When you do your export, you should not select the first login project. This works because Domino sends a token to your browser that is good for that session. Subsequent accesses will not need to login again.
I hope this helps!