For those interested, you can combine normal maps with this asset as well. I have been looking for a method without using an extensive node-tree and what-not in Blender, or it seems to lose data from either maps in the mix. I use this to create surfaces for variation.
Add another generator or copy the one you already have, choose a bump map you'd like to combine for this new group, and set the top normal map group to overlay.
You can then adjust the texture value in the Live Lighting in each group at the level you'd like it to be and - Voila! - you have combined two maps in one go without any notable loss in quality.
Here is my experimentation (not used for any good, but is just to show how it will look):
My top-most normal map (and the one set to overlay):
The one I'd like to combine it with:
And the result (after adjusting the texture levels on each Live Lighting to where I'd like them to be):