If I’m understanding correctly, the goal is to make a low-polygon mesh look like it is higher res using only a diffuse texture? That is not simple because a diffuse map contains only surface color data. In contrast a bump map is using color as a value for a faked surface offset. Normal maps do the same thing but more accurately because it’s actually three-dimensional information.
Without using either of those map inputs, you just have to fake it which is what it looks like is going on in that spotlight. In other words, the artist is painting in the shadows and highlights that would be present on the high-res mesh in such a way that the low-res mesh looks like it has more detail. That’s inevitably going to be a very manual process, not something you can easily “bake” from one mesh to another like you can with bump/normal
That said, I think helluvamesh has a good suggestion about cavity maps (you could also use ambient occlusion). You would bring this into your paint program and use it to add shadows to your final diffuse map. If you follow the notes from the spotlight, she mentions using a baked matcap from zbrush and using it in her diffuse painting process. This is probably the closest to what you’re looking to do, and I’d guess that is possible in blender somehow. But again, this is a matter of “faking” highlights and shadows caused by wrinkles and such.
The final bit of warning I will give is that the end result is always going to look like it has the exact same lighting used in the original paint since it’s built into the surface color. So if you put your model in a scene with drastically different lighting (from below, say) it’s going to look odd.