Hi @lisadv ...
I may be misunderstanding your question, but why not just use a single discussion topic, put your assignment instructions in there, and then set it as a graded discussion topic? That way, your students have the instructions right there, and you can tell them that if there are questions about the assignment, they can post them directly below the instructions? This way, you don't have to have a regular Canvas assignment and a separate discussion topic to deal with. Would this work for your needs?
Again, maybe I'm not fully understanding what you're end goal is, so I apologize in advance if I've not fully answered your question.
Hope to hear back from you on this.