Hi @jenny5 . Just as a clarification, are you looking to have a set of discussion questions that everyone in every course participates in together, a way to take a discussion question and put this in all of your courses so that each class participates in the discussion question as a class but it is the same DQ in all courses, or when you refer to accessible are you speaking about this in terms of universal design for students who may have vision/hearing trouble for example?
If you are asking for one discussion across all courses in which all students at your school can respond to the the one single discussion; then no, you cannot do that in Canvas nor would you want to. Discussions with more than about 100 participants become unimaginably difficult to manage! Believe me, I have tried in a set of cross-listed courses.
If this is not what you are asking, then please clarify as ericwerth asked, and we will do our best to help.