If you are getting an error message that indicates you cannot set an assignment due date because it is outside the official term dates, there is a way to easily over-ride this. For courses that start/end on a schedule outside of the official academic calendar (e.g. WPI Online courses), you can go into the course Settings, change the Participation to Course, and then enter in your desired start and end dates.
If you want students to have access to the course after it has officially ended, then simply leave the end-date blank.