I wanted to have multiple due dates and grades for a forum discussion. I found a work around with the "move this discussion" function.
Simply open two forums--set the maximum grade in the ratings section so they'll show up in the gradebook. You might want to restrict the second one for now.
In our case, students were to post SMART goals for their group work. We gave them a due date (as others said, it won't show up in a calendar) and rated items posted by that date. Oh and this forum was set as a Q&A so that each group responded to the instructor's prompt, this kept it to one discussion per group.
After rating the first forum, simply open each discussion--we had one for each group--and select the new forum in the "move this discussion to..." drop down menu. Click move and it will show in the new forum.
It took a few minutes to move all 18 discussions but it was the only way I found to do what I wanted. The discussions will completely disappear from the first forum and fully appear in the next.
In the new forum (I set it to a Standard forum), set a new ratings window and students will be able to see their previous posts but only the new posts/replies can be rated. Now you'll have two grade items in the gradebook. (Sadly, the initial ratings, while saved in the gradebook as a grade with the previous forum, will no longer be visible for each post after you move the discussion.)
The other solution I'd devised was to simply keep adding to the grade in the original forum with new ratings--just keep extending the rating window. Then set to sum of grades and they'll add up in the gradebook. This was a less elegant solution, but could have been less work than moving the discussions.
I would love to see the forums set with grading options for multiple grading windows. But until then, this will work for us.