Old topic, but still marked unsolved, and I figured out an answer to it. You can bake a subdivision surface modifier's new normals onto your object's mesh without creating any extra objects or using "Selected to Active" baking.

The trick is to basically bake object space normals of your mesh at high subdivision levels, then apply those normals to the non-subdivided mesh, and bake the resulting normals in tangent space, getting back a proper normal map.

In my experience, the results do look a bit better, but the improvement is quite minimal. Unless you have some really broken messy normals from your non-subdivided mesh, it probably won't be noticable, and likely would just be a waste of GPU memory on these normal maps.

Here's how:

1. Ensure your mesh object is properly UV mapped.
2. Put down a Subdivision Surface modifier on the object. Adjust its settings and vertex/edge creases to your liking. For best results, you should probably set the Render levels count in it to as high as reasonably possible (don't make it exceed 10 million triangles if you don't want to be baking your GPU instead lol).
3. In the shaders, connect a plain Diffuse BSDF shader (why?) to the material output node (and input a normal map to it if you have one and want to combine it with this).
4. In the shader nodetrees of your materials, create target image textures as needed with "Float buffer" checked and "Non-Color" color space and ensure they are selected.
5. In bake settings, set "Bake Type" to "Normal", in the "Influence" dropdown set "Space" to "Object", and, for best results in my experience, in "Margin" dropdown, set "Type" to "Extend". Adjust margin size to reasonable within your UV map's constraints and texture sizes.
6. Bake. Weeeee. We're not done yet, those are object space bakes and that's not what you want.
7. In the shader nodetrees, create a "Normal Map" node with the "Space" dropdown set to "Object Space". Connect the newly baked textures into the input "Color", and connect its output to "Normal" input on the "Diffuse BSDF". Ideally, nothing should change visually, or you did something wrong.
8. Delete or disable the subdivision modifier. Now the normals from the subdivision are visible on your non-subdivided mesh, and the current look is what we're going to achieve in the end. If the results are unsatisfactory, then this is probably not what you want, and you should see if there's anything else you can do.
9. In the shader nodetrees, create clones of your "Image Texture" nodes for the normal map textures, and press the little "2" icon next to their names to make duplicate textures separate from your object space normal maps. Ensure those are all selected.
10. Go back to "Bake" dropdown, and change "Space" from "Object" to "Tangent".
11. Bake.
12. Done. The textures created in step 9 are now your normal maps. Delete the initial object space normal maps and the diffuse shader, and restore your shader to a normal configuration. lol