It’s possible to put some content above the timeline, for example, Car Talk has some suggestions above their timeline here: Battery finally dead - Maintenance/Repairs - Car Talk Community (but note that it’s not fixed on scroll)
That was done in their theme by adding content to the above-timeline plugin outlet in Discourse. We don’t plan on adding ad space above/below the timeline to the ad plugin by default, so it needs to be done in a custom plugin or theme.
Using plugin outlets in themes is covered a little here: Beginner's guide to developing Discourse Themes