But this course didn’t feature hi-poly model baking to lo-poly…
Maybe you’re not referring to this course specifically, but just as an example. So perhaps you’re trying to add additional details to a normal map via painting bump in the viewport. I think that can technically be done to a degree, but not with Blender 2.8 afaik. Try it with Blender Render in 2.79.
What you need to do is setup separate bump and normal textures in your material. Set the baked normal to influence the Normal channel (in the geometry section of the texture influence panel) then add a second “bump” map (black and white) to also affect the normal channel. Then you can paint white and black values into that bump map and they should appear combined with your normal map.
Understand though they they could end up fighting each other. It’s a bit of a tricky thing you’re after. Good luck with it though!
